Ccna Presentation

Data Networks
Sharing data through the use of floppy disks is not an efficient
or cost-effective manner in which to operate businesses.

Businesses needed a solution that would successfully
address the following three problems:
• How to avoid duplication of equipment and resources
• How to communicate efficiently
• How to set up and manage a network

Businesses realized that networking technology could
increase productivity while saving money.

3

Networking Devices

Equipment that connects directly to a network segment is
referred to as a device.

These devices are broken up into two classifications.
• end-user devices
• network devices

End-user devices include computers, printers, scanners, and
other devices that provide services directly to the user.

Network devices include all the devices that connect the end-
user devices together to allow them to communicate.
4

Network Interface Card
A network interface card (NIC) is a printed circuit board
that provides network communication capabilities to and
from a personal computer. Also called a LAN adapter.

5

Networking Device Icons

6

Repeater
A repeater is a network device used to regenerate a signal.
Repeaters regenerate analog or digital signals distorted by
transmission loss due to attenuation. A repeater does not
perform intelligent routing.

7

Hub
Hubs concentrate
connections. In other
words, they take a group of
hosts and allow the network
to see them as a single unit.

This is done
passively, without any other
effect on the data
transmission.

Active hubs not only
concentrate hosts, but they 8
also regenerate signals.

Bridge
Bridges convert network transmission data formats as well as
perform basic data transmission management. Bridges, as
the name implies, provide connections between LANs. Not
only do bridges connect LANs, but they also perform a check
on the data to determine whether it should cross the bridge or
not. This makes each part of the network more efficient.

9

Workgroup Switch

Workgroup switches add
more intelligence to data
transfer management.

Switches can determine
whether data should remain
on a LAN or not, and they
can transfer the data to the
connection that needs that
data.

10

Router
Routers have all capabilities of the previous devices. Routers
can regenerate signals, concentrate multiple
connections, convert data transmission formats, and manage
data transfers.They can also connect to a WAN, which allows
them to connect LANs that are separated by great distances.

11

―The Cloud‖
The cloud is used in diagrams to represent where the
connection to the internet is.
It also represents all of the devices on the internet.

12

Network Topologies
Network topology defines the structure of the network.

One part of the topology definition is the physical
topology, which is the actual layout of the wire or media.

The other part is the logical topology,which defines how the
media is accessed by the hosts for sending data.

13

Physical Topologies

14

Bus Topology
A bus topology uses a single backbone cable that is
terminated at both ends.

All the hosts connect directly to this backbone.

15

Ring Topology
A ring topology connects one host to the next and the last host
to the first.

This creates a physical ring of cable.

16

Star Topology
A star topology connects all cables to a central point of
concentration.

17

Extended Star Topology
An extended star topology links individual stars together by
connecting the hubs and/or switches.This topology can extend
the scope and coverage of the network.

18

Hierarchical Topology

A hierarchical topology is similar to an extended star.

19

Mesh Topology
A mesh topology is implemented to provide as much
protection as possible from interruption of service.
Each host has its own connections to all other hosts.
Although the Internet has multiple paths to any one
location, it does not adopt the full mesh topology.

20

LANs, MANs, & WANs

One early solution was the creation of local-area network
(LAN) standards which provided an open set of guidelines for
creating network hardware and software, making equipment
from different companies compatible.

What was needed was a way for information to move
efficiently and quickly, not only within a company, but also
from one business to another.

The solution was the creation of metropolitan-area networks
(MANs) and wide-area networks (WANs).

21

Examples of Data Networks

22

Wireless LAN Organizations
and Standards
In cabled networks, IEEE is the prime issuer of standards for
wireless networks. The standards have been created within the
framework of the regulations created by the Federal
Communications Commission (FCC).

A key technology contained within the 802.11 standard is Direct
Sequence Spread Spectrum (DSSS).

24

Cellular Topology for Wireless

25

SANs

A SAN is a dedicated, high-
performance network used to
move data between servers
and storage resources.

Because it is a
separate, dedicated
network, it avoids any traffic
conflict between clients and
servers.
27

Virtual Private Network
A VPN is a private network that is constructed within a public network
infrastructure such as the global Internet. Using VPN, a telecommuter
can access the network of the company headquarters through the
Internet by building a secure tunnel between the telecommuter’s PC
and a VPN router in the headquarters.

28

Measuring Bandwidth

30

Why do we need the OSI Model?

To address the problem of networks increasing in size
and in number, the International Organization for
Standardization (ISO) researched many network
schemes and recognized that there was a need to
create a network model that would help network
builders implement networks that could communicate
and work together and therefore, released the OSI
reference model in 1984.

32

Don’t Get Confused.

ISO - International Organization for Standardization

OSI - Open System Interconnection

IOS - Internetwork Operating System

The ISO created the OSI to make the IOS more
efficient. The ―ISO‖ acronym is correct as shown.

To avoid confusion, some people say ―International
Standard Organization.‖
33

The OSI Reference Model
7 Application The OSI Model will be
used throughout your
6 Presentation
entire networking
5 Session career!
4 Transport
3 Network
Memorize it!
2 Data Link
1 Physical

34

Layer 7 - The Application Layer
7 Application This layer deal with
networking applications.
6 Presentation
5 Session Examples:
4 Transport  Email
 Web browsers
3 Network
2 Data Link PDU - User Data
1 Physical

35

Layer 6 - The Presentation Layer
7 Application This layer is responsible
for presenting the data in
6 Presentation
the required format which
5 Session may include:
4 Transport  Encryption
 Compression
3 Network
2 Data Link PDU - Formatted Data
1 Physical

36

Layer 5 - The Session Layer
7 Application This layer establishes,
manages, and terminates
6 Presentation
sessions between two
5 Session communicating hosts.
4 Transport
Example:
3 Network  Client Software
2 Data Link ( Used for logging in)

1 Physical PDU - Formatted Data

37

Layer 4 - The Transport Layer
7 Application This layer breaks up the
data from the sending host
6 Presentation
and then reassembles it in
5 Session the receiver.
4 Transport
It also is used to insure
3 Network reliable data transport
2 Data Link across the network.
1 Physical
PDU - Segments

38

Layer 3 - The Network Layer
7 Application Sometimes referred to as the
―Cisco Layer‖.
6 Presentation
5 Session Makes ―Best Path
4 Transport Determination‖ decisions
based on logical addresses
3 Network (usually IP addresses).
2 Data Link
PDU - Packets
1 Physical

39

Layer 2 - The Data Link Layer
7 Application This layer provides reliable
transit of data across a
6 Presentation
physical link.
5 Session
4 Transport Makes decisions based on
physical addresses (usually
3 Network MAC addresses).
2 Data Link
PDU - Frames
1 Physical

40

Layer 1 - The Physical Layer
This is the physical media
7 Application through which the data,
6 Presentation represented as electronic
signals, is sent from the
5 Session
source host to the
4 Transport destination host.
3 Network
Examples:
2 Data Link  CAT5 (what we have)
1 Physical  Coaxial (like cable TV)
 Fiber optic

PDU - Bits 41

OSI Model Analogy
Application Layer - Source Host

After riding your new bicycle a few times in
NewYork, you decide that you want to give it to a
friend who lives in Munich,Germany. 42

OSI Model Analogy
Presentation Layer - Source Host

Make sure you have the proper directions to
disassemble and reassemble the bicycle.
43

OSI Model Analogy
Session Layer - Source Host

Call your friend and make sure you have his
correct address.
44

OSI Model Analogy
Transport Layer - Source Host

Disassemble the bicycle and put different pieces
in different boxes. The boxes are labeled
―1 of 3‖, ―2 of 3‖, and ―3 of 3‖. 45

OSI Model Analogy
Network Layer - Source Host

Put your friend's complete mailing address (and
yours) on each box.Since the packages are too
big for your mailbox (and since you don’t have
enough stamps) you determine that you need to
go to the post office. 46

OSI Model Analogy
Data Link Layer – Source Host

NewYork post office takes possession of the
boxes.
47

OSI Model Analogy
Physical Layer - Media

The boxes are flown from USA to Germany.

48

OSI Model Analogy
Data Link Layer - Destination

Munich post office receives your boxes.

49

OSI Model Analogy
Network Layer - Destination

Upon examining the destination address,
Munich post office determines that your
boxes should be delivered to your written
home address. 50

OSI Model Analogy
Transport Layer - Destination

Your friend calls you and tells you he got all 3
boxes and he is having another friend named
BOB reassemble the bicycle. 51

OSI Model Analogy
Session Layer - Destination

Your friend hangs up because he is done talking
to you.
52

OSI Model Analogy
Presentation Layer - Destination

BOB is finished and ―presents‖ the bicycle to
your friend. Another way to say it is that your
friend is finally getting him ―present‖. 53

OSI Model Analogy
Application Layer - Destination

Your friend enjoys riding his new bicycle in
Munich.
54

Host Layers
7 Application These layers
only exist in the
6 Presentation
source and
5 Session destination host
4 Transport computers.

3 Network
2 Data Link
1 Physical

55

Media Layers
7 Application
6 Presentation
5 Session
4 Transport
These layers manage
3 Network
the information out in
2 Data Link the LAN or WAN
1 Physical between the source
and destination hosts.

56

Data Flow Through a Network

59

LAN Physical Layer
Various symbols are used to represent media types.

The function of media is to carry a flow of information
through a LAN.Networking media are considered
Layer 1, or physical layer, components of LANs.

Each media has advantages and disadvantages.
Some of the advantage or disadvantage comparisons
concern:
• Cable length
• Cost
• Ease of installation
• Susceptibility to interference
Coaxial cable, optical fiber, and even free space can
carry network signals. However, the principal medium
that will be studied is Category 5 unshielded twisted-
pair cable (Cat 5 UTP)
61

Unshielded Twisted Pair (UTP) Cable

62

UTP Implementation
EIA/TIA specifies an RJ-45 connector for UTP cable.
The RJ-45 transparent end connector shows eight colored wires.
Four of the wires carry the voltage and are considered ―tip‖ (T1 through T4).
The other four wires are grounded and are called ―ring‖ (R1 through R4).
The wires in the first pair in a cable or a connector are designated as T1 & R1

63

Connection Media
The registered jack (RJ-45) connector and jack are the most
common.

In some cases the type of connector on a network interface
card (NIC) does not match the media that it needs to connect
to.

The attachment unit interface (AUI) connector allows different
media to connect when used with the appropriate transceiver.

A transceiver is an adapter that converts one type of
connection to another.
64

Ethernet Standards

The Ethernet standard specifies that each of the pins on an
RJ-45 connector have a particular purpose. A NIC transmits
signals on pins 1 & 2, and it receives signals on pins 3 & 6.

65

Remember…

A straight-thru cable has T568B on both ends. A crossover (or
cross-connect) cable has T568B on one end and T568A on the
other. A console cable had T568B on one end and reverse T568B
on the other, which is why it is also called a rollover cable.

66

Straight-Thru or Crossover

Use straight-through cables for the following cabling:
• Switch to router
• Switch to PC or server
• Hub to PC or server
Use crossover cables for the following cabling:
• Switch to switch
• Switch to hub
• Hub to hub
• Router to router
• PC to PC
• Router to PC 67

Sources of Noise on Copper Media

Noise is any electrical energy on the
transmission cable that makes it difficult for a
receiver to interpret the data sent from the
transmitter. TIA/EIA-568-B certification of a cable
now requires testing for a variety of types of
noise.Twisted-pair cable is designed to take
advantage of the effects of crosstalk in order to
minimize noise. In twisted-pair cable, a pair of
wires is used to transmit one signal.The wire pair
is twisted so that each wire experiences similar
crosstalk. Because a noise signal on one wire
will appear identically on the other wire, this
noise be easily detected and filtered at
receiver.Twisting one pair of wires in a cable also
helps to reduce crosstalk of data or noise signals
from adjacent wires. 68

Shielded Twisted Pair (STP) Cable

69

Fiber Optic Cable

71

Fiber Optic Connectors
Connectors are attached to the fiber ends so that the fibers can
be connected to the ports on the transmitter and receiver.
The type of connector most commonly used with multimode fiber
is the Subscriber Connector (SC connector).On single-mode
fiber, the Straight Tip (ST) connector is frequently used

72

Fiber Optic Patch Panels
Fiber patch panels similar to the patch panels used with copper
cable.

73

Cable Specifications

10BASE-T
The T stands for twisted pair.
10BASE5
The 5 represents the fact that a signal can travel for approximately
500 meters 10BASE5 is often referred to as Thicknet.
10BASE2
The 2 represents the fact that a signal can travel for approximately
200 meters 10BASE2 is often referred to as Thinnet.

All 3 of these specifications refer to the speed of transmission at 10
Mbps and a type of transmission that is baseband, or digitally
interpreted. Thinnet and Thicknet are actually a type of
networks, while 10BASE2 & 10BASE5 are the types of cabling used in
these networks. 74

Ethernet Media Connector Requirements

75

LAN Physical Layer Implementation

76

Ethernet in the Campus

77

WAN Physical Layer

78

WAN Serial Connection Options

79

Serial Implementation of DTE & DCE
When connecting directly to a service provider, or to a
device such as a CSU/DSU that will perform signal
clocking, the router is a DTE and needs a DTE serial cable.
This is typically the case for routers.

80

Back-to-Back Serial Connection

When
performing a
back-to-back
router scenario
in a test
environment, on
e of the routers
will be a DTE
and the other
will be a DCE.

81

Repeater
A repeater is a network device used to regenerate a signal.
Repeaters regenerate analog or digital signals distorted by
transmission loss due to attenuation.Repeater is a Physical
Layer device

82

The 4 Repeater Rule
The Four Repeater Rule for 10-Mbps Ethernet should be
used as a standard when extending LAN segments.

This rule states that no more than four repeaters
can be used between hosts on a LAN.

This rule is used to limit latency added to frame travel by
each repeater.

83

Hub

Hubs concentrate
connections.In other
words, they take a group of
hosts and allow the network
to see them as a single unit.
Hub is a physical layer
device.

84

Network Interface Card
The function of a NIC is to connect a host device to the network medium.

A NIC is a printed circuit board that fits into the expansion slot on the motherboard or
peripheral device of a computer. The NIC is also referred to as a network adapter.

NICs are considered Data Link Layer devices because each NIC carries a
unique code called a MAC address.

85

MAC Address
MAC address is 48 bits in length and expressed as twelve hexadecimal
digits.MAC addresses are sometimes referred to as burned-in addresses
(BIA) because they are burned into read-only memory (ROM) and are
copied into random-access memory (RAM) when the NIC initializes.

86

Bridge
Bridges are Data Link layer devices.Connected host
addresses are learned and stored on a MAC address
table.Each bridge port has a unique MAC address

87

Bridging Graphic

89

Switch

Switches are Data Link
layer devices.

Each Switch port has a
unique MAC address.

Connected host MAC
addresses are learned and
stored on a MAC address
table.

90

Switching Modes

cut-through
A switch starts to transfer the frame as soon as the destination MAC
address is received. No error checking is available.
Must use synchronous switching.

store-and-forward
At the other extreme, the switch can receive the entire frame before
sending it out the destination port. This gives the switch software an
opportunity to verify the Frame Check Sum (FCS) to ensure that the frame
was reliably received before sending it to the destination.
Must be used with asynchronous switching.

fragment-free
A compromise between the cut-through and store-and-forward modes.
Fragment-free reads the first 64 bytes, which includes the frame
header, and switching begins before the entire data field and checksum
are read.
91

Full Duplex

Another capability emerges when only two nodes are connected. In a network that
uses twisted-pair cabling, one pair is used to carry the transmitted signal from one
node to the other node. A separate pair is used for the return or received signal. It is
possible for signals to pass through both pairs simultaneously. The capability of
communication in both directions at once is known as full duplex.

92

Switches – MAC Tables

93

Switches – Parallel Communication

94

Microsegmentation

A switch is simply a bridge with many ports. When only one node is connected to a
switch port, the collision domain on the shared media contains only two nodes.
The two nodes in this small segment, or collision domain, consist of the switch port
and the host connected to it. These small physical segments are called micro
segments.

95

Peer-to-Peer Network
In a peer-to-peer network, networked computers act as equal partners, or peers.

As peers, each computer can take on the client function or the server function.

At one time, computer A may make a request for a file from computer B, which
responds by serving the file to computer A. Computer A functions as client, while B
functions as the server. At a later time, computers A and B can reverse roles.

In a peer-to-peer network, individual users control their own resources. Peer-to-
peer networks are relatively easy to install and operate. As networks grow, peer-to-
peer relationships become increasingly difficult to coordinate.

96

Client/Server Network
In a client/server arrangement, network services are located on a dedicated
computer called a server.

The server responds to the requests of clients.

The server is a central computer that is continuously available to respond to
requests from clients for file, print, application, and other services.

Most network operating systems adopt the form of a client/server relationship.

97

Why Another Model?
Although the OSI reference model is universally
recognized, the historical and technical open standard
of the Internet is Transmission Control Protocol /
Internet Protocol (TCP/IP).

The TCP/IP reference model and the TCP/IP protocol
stack make data communication possible between any
two computers, anywhere in the world, at nearly the
speed of light.

The U.S. Department of Defense (DoD) created the
TCP/IP reference model because it wanted a network
that could survive any conditions, even a nuclear war.
99

Don’t Confuse the Models

7 Application
6 Presentation Application
5 Session
4 Transport Transport
3 Network Internet
2 Data Link Network
1 Physical Access 100

2 Models
Side-By-Side
7 Application
6 Presentation Application
5 Session
4 Transport Transport
3 Network Internet
2 Data Link Network
1 Physical Access
101

The Application Layer
The application
layer of the
TCP/IP model
handles high-
level
protocols, issue
s of
representation,
encoding, and
dialog control.
102

The Transport Layer

The transport layer provides transport services from
the source host to the destination host. It constitutes
a logical connection between these endpoints of the
network. Transport protocols segment and
reassemble upper-layer applications into the same
data stream between endpoints.
The transport layer data stream provides end-to-end
transport services. 103

The Internet Layer
The purpose of the Internet layer is to
select the best path through the network for
packets to travel. The main protocol that
functions at this layer is the Internet
Protocol (IP). Best path determination and
packet switching occur at this layer.

104

The Network Access Layer
The network access layer is also called the host-to-
network layer. It the layer that is concerned with all of the
issues that an IP packet requires to actually make a
physical link to the network media. It includes LAN and
WAN details, and all the details contained in the OSI
physical and data-link layers. NOTE: ARP & RARP work
at both the Internet and Network Access Layers.

105

Comparing TCP/IP & OSI Models

NOTE: TCP/IP transport layer using UDP does not always guarantee
reliable delivery of packets as the transport layer in the OSI model does.

106

Introduction to the Transport Layer

The primary duties of the transport layer, Layer 4 of the OSI
model, are to transport and regulate the flow of information from
the source to the destination, reliably and accurately.

End-to-end control and reliability are provided by sliding
windows, sequencing numbers, and acknowledgments.

107

More on The Transport Layer

The transport layer provides transport services from the
source host to the destination host.

It establishes a logical connection between the endpoints of
the network.
• Transport services include the following basic services:
• Segmentation of upper-layer application data
• Establishment of end-to-end operations
• Transport of segments from one end host to another
end host
• Flow control provided by sliding windows
• Reliability provided by sequence numbers and
acknowledgments 108

Flow Control
As the transport layer sends data segments, it tries to ensure that data is not lost.
A receiving host that is unable to process data as quickly as it arrives could be a
cause of data loss.

Flow control avoids the problem of a transmitting host overflowing the buffers in
the receiving host.

109

3-Way Handshake
TCP requires connection establishment before data transfer begins.
For a connection to be established or initialized, the two hosts must
synchronize their Initial Sequence Numbers (ISNs).

110

Basic Windowing
Data packets must be
delivered to the
recipient in the same
order in which they
were transmitted to
have a
reliable, connection-
oriented data transfer.
The protocol fails if
any data packets are
lost, damaged, duplic
ated, or received in a
different order.
An easy solution is to
have a recipient
acknowledge the
receipt of each packet
before the next
packet is sent.
111

Sliding Window
with Different Window Sizes

113

TCP Sequence & Acknowledgement

114

TCP
Transmission Control Protocol (TCP) is a connection-oriented Layer 4
protocol that provides reliable full-duplex data transmission.

TCP is part of the TCP/IP protocol stack. In a connection-oriented
environment, a connection is established between both ends before the
transfer of information can begin.
TCP is responsible for breaking messages into segments, reassembling
them at the destination station, resending anything that is not received,
and reassembling messages from the segments.TCP supplies a virtual
circuit between end-user applications.

The protocols that use TCP include:
• FTP (File Transfer Protocol)
• HTTP (Hypertext Transfer Protocol)
• SMTP (Simple Mail Transfer Protocol)
• Telnet 115

TCP Segment Format

116

UDP
User Datagram Protocol (UDP) is the connectionless transport protocol
in the TCP/IP protocol stack.

UDP is a simple protocol that exchanges datagrams, without
acknowledgments or guaranteed delivery. Error processing and
retransmission must be handled by higher layer protocols.

UDP uses no windowing or acknowledgments so reliability, if needed, is
provided by application layer protocols. UDP is designed for applications
that do not need to put sequences of segments together.

The protocols that use UDP include:
• TFTP (Trivial File Transfer Protocol)
• SNMP (Simple Network Management Protocol)
• DHCP (Dynamic Host Control Protocol)
• DNS (Domain Name System) 117

UDP Segment Format

118

Well Known Port Numbers
The following port numbers should be memorized:
NOTE:
The curriculum forgot to mention one of the most important port numbers.
Port 80 is used for HTTP or WWW protocols. (Essentially access to the internet.)

119

SNMP – Managed Network

121

Base 2 Number System

101102 = (1 x 24 = 16) + (0 x 23 = 0) + (1 x 22 = 4) +
(1 x 21 = 2) + (0 x 20 = 0) = 22

123

Converting Decimal to Binary

Convert 20110 to binary:
201 / 2 = 100 remainder 1
100 / 2 = 50 remainder 0
50 / 2 = 25 remainder 0
25 / 2 = 12 remainder 1
12 / 2 = 6 remainder 0
When the quotient is 0, take all the remainders in
reverse order for your answer: 20110 = 110010012
124

Network and Host Addressing
Using the IP address of the
destination network, a router can
deliver a packet to the correct
network.

When the packet arrives at a
router connected to the
destination network, the router
uses the IP address to locate the
particular computer connected to
that network.
Accordingly, every IP address has
two parts. 126

Network Layer Communication Path

A router forwards packets from the originating network to the
destination network using the IP protocol. The packets must
include an identifier for both the source and destination networks.

127

Internet Addresses
IP Addressing is a hierarchical structure.An IP address combines two
identifiers into one number. This number must be a unique
number, because duplicate addresses would make routing
impossible.The first part identifies the system's network address.The
second part, called the host part, identifies which particular machine
it is on the network.

128

IP Address Classes

IP addresses are divided into classes to define the
large, medium, and small networks.

Class A addresses are assigned to larger networks.
Class B addresses are used for medium-sized networks, &
Class C for small networks.

129

Identifying Address Classes

130

Address Class Prefixes
To accommodate different size networks and aid in classifying these networks, IP
addresses are divided into groups called classes.This is classful addressing.

131

Network and Host Division
Each complete 32-bit IP address is broken down into a network part
and a host part. A bit or bit sequence at the start of each address
determines the class of the address. There are 5 IP address classes.

132

Class A Addresses

The Class A address was designed to support extremely large
networks, with more than 16 million host addresses available.
Class A IP addresses use only the first octet to indicate the
network address. The remaining three octets provide for host
addresses.

133

Class B Addresses

The Class B address was designed to support the needs of
moderate to large-sized networks.A Class B IP address uses
the first two of the four octets to indicate the network address.
The other two octets specify host addresses.

134

Class C Addresses

The Class C address space is the most commonly used of the
original address classes.This address space was intended to
support small networks with a maximum of 254 hosts.

135

Class D Addresses

The Class D address class was created to enable multicasting in an
IP address. A multicast address is a unique network address that
directs packets with that destination address to predefined groups of
IP addresses. Therefore, a single station can simultaneously transmit
a single stream of data to multiple recipients.

136

Class E Addresses

A Class E address has been defined. However, the Internet
Engineering Task Force (IETF) reserves these addresses for its
own research. Therefore, no Class E addresses have been
released for use in the Internet.

137

IP Address Ranges

The graphic below shows the IP address range of the first octet
both in decimal and binary for each IP address class.

138

IPv4

As early as 1992, the Internet Engineering
Task Force (IETF) identified two specific
concerns: Exhaustion of the
remaining, unassigned IPv4 network
addresses and the increase in the size of
Internet routing tables.

Over the past two decades, numerous
extensions to IPv4 have been developed.
Two of the more important of these are
subnet masks and classless interdomain
routing (CIDR).

139

Finding the Network Address with ANDing
By ANDing the Host address of 192.168.10.2 with 255.255.255.0
(its network mask) we obtain the network address of 192.168.10.0

140

Network Address

141

Broadcast Address

142

Network/Broadcast Addresses
at the Binary Level
An IP address that has binary 0s in all host bit positions is
reserved for the network address, which identifies the network.
An IP address that has binary 1s in all host bit positions is
reserved for the broadcast address, which is used to send data
to all hosts on the network. Here are some examples:

Class Network Address Broadcast Address

A 100.0.0.0 100.255.255.255

B 150.75.0.0 150.75.255.255

C 200.100.50.0 200.100.50.255 143

Public IP Addresses
Unique addresses are required for each device on a network.

Originally, an organization known as the Internet Network Information
Center (InterNIC) handled this procedure.

InterNIC no longer exists and has been succeeded by the Internet Assigned
Numbers Authority (IANA).

No two machines that connect to a public network can have the same IP
address because public IP addresses are global and standardized.

All machines connected to the Internet agree to conform to the system.

Public IP addresses must be obtained from an Internet service provider
(ISP) or a registry at some expense.
144

Private IP Addresses

Private IP addresses are another solution to the problem of the
impending exhaustion of public IP addresses.As mentioned, public
networks require hosts to have unique IP addresses.

However, private networks that are not connected to the Internet may
use any host addresses, as long as each host within the private
network is unique.

145

Mixing Public and
Private IP Addresses
Private IP addresses can be intermixed, as shown in the graphic, with
public IP addresses.This will conserve the number of addresses used for
internal connections. Connecting a network using private addresses to
the Internet requires translation of the private addresses to public
addresses. This translation process is referred to as Network Address
Translation (NAT).

146

Introduction to Subnetting
Subnetting a network means to use the subnet mask to divide the
network and break a large network up into smaller, more efficient and
manageable segments, or subnets.

With subnetting, the network is not limited to the default Class A, B, or
C network masks and there is more flexibility in the network design.

Subnet addresses include the network portion, plus a subnet field and
a host field.The ability to decide how to divide the original host portion
into the new subnet and host fields provides addressing flexibility for
the network administrator.

147

The 32-Bit
Binary IP Address

148

Numbers That Show Up In
Subnet Masks (Memorize Them!)

149

Addressing with Subnetworks

150

Obtaining an Internet Address

151

Static Assignment of an IP Address

Static assignment
works best on small
networks.

The administrator
manually assigns and
tracks IP addresses
for each
computer, printer, or
server on the intranet.

Network
printers, application
servers, and routers
should be assigned
static IP addresses. 152

ARP
(Address Resolution Protocol)
Host A
ARP Request - Broadcast to all hosts
SIEMENS
NIXDORF
„What is the hardware address for IP address 128.0.10.4?―

ARP Reply

SIEMENS
NIXDORF
SIEMENS
NIXDORF

Host B
IP Address: 128.0.10.4
HW Address: 080020021545

153
Fig. 32 How does ARP work? (TI1332EU02TI_0004 The Network Layer, 47)

154
Fig. 33 The ARP command (TI1332EU02TI_0004 The Network Layer, 47)

1 Network = 1 Broadcast Domain

A B host B would reply

Broadcast: ARP request

2 Networks = 2 Broadcast Domains

A B no one would reply
Router

Broadcast: ARP request

155
Fig. 34 Proxy-ARP concept (TI1332EU02TI_0004 The Network Layer, 49)

A

B

A
B

Router R

I take care, to forward
IP packets to B

Broadcast Message to all:
If your IP address matches ―B‖ Yes, I know the destination
then please tell me your network, let me give you my
Ethernet address Ethernet address

156

RARP

Reverse Address Resolution Protocol (RARP) associates a known MAC addresses
with an IP addresses.

A network device, such as a diskless workstation, might know its MAC address but not
its IP address. RARP allows the device to make a request to learn its IP address.
Devices using RARP require that a RARP server be present on the network to answer
RARP requests.

157

BootP
The bootstrap protocol (BOOTP) operates in a client-server environment and only
requires a single packet exchange to obtain IP information.

However, unlike RARP, BOOTP packets can include the IP address, as well as
the address of a router, the address of a server, and vendor-specific information.

One problem with BOOTP, however, is that it was not designed to provide
dynamic address assignment. With BOOTP, a network administrator creates a
configuration file that specifies the parameters for each device.The administrator
must add hosts and maintain the BOOTP database.

Even though the addresses are dynamically assigned, there is still a one to one
relationship between the number of IP addresses and the number of hosts.

This means that for every host on the network there must be a BOOTP profile
with an IP address assignment in it. No two profiles can have the same IP
address.
158

DHCP

Dynamic host configuration protocol (DHCP) is the successor to BOOTP.

Unlike BOOTP, DHCP allows a host to obtain an IP address dynamically without the
network administrator having to set up an individual profile for each device.

All that is required when using DHCP is a defined range of IP addresses on a DHCP
server.As hosts come online, they contact the DHCP server and request an address.

The DHCP server chooses an address and leases it to that host.

With DHCP, the entire network configuration of a computer can be obtained in one
message.

This includes all of the data supplied by the BOOTP message, plus a leased IP
address and a subnet mask.

The major advantage that DHCP has over BOOTP is that it allows users to be mobile.
159

Introduction to Routers
A router is a special type of computer. It has the same basic components as a
standard desktop PC. However, routers are designed to perform some very specific
functions. Just as computers need operating systems to run software
applications, routers need the Internetwork Operating System software (IOS) to run
configuration files. These configuration files contain the instructions and parameters
that control the flow of traffic in and out of the routers. The many parts of a router are
shown below:

161

RAM
Random Access Memory, also called dynamic RAM (DRAM)

RAM has the following characteristics and functions:

• Stores routing tables
• Holds ARP cache
• Holds fast-switching cache
• Performs packet buffering (shared RAM)
• Maintains packet-hold queues
• Provides temporary memory for the configuration file of
the router while the router is powered on
• Loses content when router is powered down or restarted

162

NVRAM
Non-Volatile RAM

NVRAM has the following characteristics and functions:

• Provides storage for the startup configuration file
• Retains content when router is powered down or
restarted

163

Flash
Flash memory has the following characteristics and
functions:

• Holds the operating system image (IOS)
• Allows software to be updated without
removing and replacing chips on the processor
• Retains content when router is powered down
or restarted
• Can store multiple versions of IOS software

Is a type of electronically erasable, programmable
ROM (EEPROM) 164

ROM
Read-Only Memory

ROM has the following characteristics and functions:

• Maintains instructions for power-on self test
(POST) diagnostics
• Stores bootstrap program and basic operating
system software
• Requires replacing pluggable chips on the
motherboard for software upgrades

165

Interfaces
Interfaces have the following characteristics and functions:

• Connect router to network for frame entry and exit
• Can be on the motherboard or on a separate module

Types of interfaces:

• Ethernet
• Fast Ethernet
• Serial
• Token ring
• ISDN BRI
• Loopback
• Console
• Aux 166

Internal Components of a 2600 Router

167

External Components of a 2600 Router

168

External Connections

169

Fixed Interfaces
When cabling routers for serial connectivity, the routers will either have
fixed or modular ports. The type of port being used will affect the syntax
used later to configure each interface. Interfaces on routers with fixed
serial ports are labeled for port type and port number.

170

Modular Serial Port Interfaces
Interfaces on routers with modular serial ports are labeled for port type, slot, and port
number.The slot is the location of the module.To configure a port on a modular card, it is
necessary to specify the interface using the syntax ―port type slot number/port number.‖ Use
the label ―serial 0/1,‖ when the interface is serial, the slot number where the module is
installed is slot 0, and the port that is being referenced is port 1.

171

Routers & DSL Connections
The Cisco 827 ADSL router has one asymmetric digital
subscriber line (ADSL) interface. To connect a router for DSL
service, use a phone cable with RJ-11 connectors. DSL works
over standard telephone lines using pins 3 and 4 on a
standard RJ-11 connector.

172

Computer/Terminal Console Connection

173

Modem Connection to Console/Aux Port

174

HyperTerminal Session Properties

175

Establishing a
HyperTerminal Session
Take the following steps
to connect a terminal to
the console port on the
router:

First, connect the
terminal using the RJ-45
to RJ-45 rollover cable
and an RJ-45 to DB-9 or
RJ-45 to DB-25 adapter.

Then, configure the
terminal or PC terminal
emulation software for
9600 baud, 8 data bits,
no parity, 1 stop bit, and
no flow control.
176

Cisco IOS
Cisco technology is built around the Cisco
Internetwork Operating System (IOS), which is the
software that controls the routing and switching
functions of internetworking devices.

A solid understanding of the IOS is essential for a
network administrator.

177

The Purpose of Cisco IOS
As with a computer, a router or switch cannot function without
an operating system. Cisco calls its operating system the
Cisco Internetwork Operating System or Cisco IOS.

It is the embedded software architecture in all of the Cisco
routers and is also the operating system of the Catalyst
switches.

Without an operating system, the hardware does not have any
capabilities.

The Cisco IOS provides the following network services:
• Basic routing and switching functions
• Reliable and secure access to networked resources
• Network scalability 178

Router Command Line
Interface

179

Setup Mode
Setup is not intended as the mode for entering complex protocol features in the
router. The purpose of the setup mode is to permit the administrator to install a
minimal configuration for a router, unable to locate a configuration from another
source.

In the setup mode, default answers appear in square brackets [ ] following the
question. Press the Enter key to use these defaults.

During the setup process, Ctrl-C can be pressed at any time to terminate the
process. When setup is terminated using Ctrl-C, all interfaces will be
administratively shutdown.

When the configuration process is completed in setup mode, the following options
will be displayed:

[0] Go to the IOS command prompt without saving this config.
[1] Return back to the setup without saving this config.
[2] Save this configuration to nvram and exit.
Enter your selection [2]: 180

Operation of Cisco IOS Software
The Cisco IOS devices have three distinct operating environments or
modes:
• ROM monitor
• Boot ROM
• Cisco IOS

The startup process of the router normally loads into RAM and executes
one of these operating environments. The configuration register setting can
be used by the system administrator to control the default start up mode for
the router.

To see the IOS image and version that is running, use the show version
command, which also indicates the configuration register setting.

181

IOS File System Overview

182

Initial Startup of Cisco Routers
A router initializes by loading the bootstrap, the operating system, and a
configuration file.

If the router cannot find a configuration file, it enters setup mode.

Upon completion of the setup mode a backup copy of the configuration file
may be saved to nonvolatile RAM (NVRAM).

The goal of the startup routines for Cisco IOS software is to start the router
operations. To do this, the startup routines must accomplish the following:
• Make sure that the router hardware is tested and functional.
• Find and load the Cisco IOS software.
• Find and apply the startup configuration file or enter the setup
mode.

When a Cisco router powers up, it performs a power-on self test (POST).
During this self test, the router executes diagnostics from ROM on all
hardware modules. 183

After the Post…
After the POST, the following events occur as the router initializes:

Step 1
The generic bootstrap loader in ROM executes. A bootstrap is a simple set of
instructions that tests hardware and initializes the IOS for operation.

Step 2
The IOS can be found in several places. The boot field of the configuration register
determines the location to be used in loading the IOS. If the boot field indicates a
flash or network load, boot system commands in the configuration file indicate the
exact name and location of the image.

Step 3
The operating system image is loaded.

Step 4
The configuration file saved in NVRAM is loaded into main memory and executed
one line at a time. The configuration commands start routing processes, supply
addresses for interfaces, and define other operating characteristics of the router.

Step 5
If no valid configuration file exists in NVRAM, the operating system searches for an
184
available TFTP server. If no TFTP server is found, the setup dialog is initiated.

Step in Router Initialization

185

Router LED Indicators
Cisco routers use LED indicators to provide status information.
Depending upon the Cisco router model, the LED indicators will
vary. An interface LED indicates the activity of the corresponding
interface. If an LED is off when the interface is active and the
interface is correctly connected, a problem may be indicated. If an
interface is extremely busy, its LED will always be on. The green OK
LED to the right of the AUX port will be on after the system initializes
correctly.

186

Enhanced
Cisco IOS Commands

187

The show version Command
The show version command displays information about the Cisco IOS
software version that is currently running on the router. This includes the
configuration register and the boot field settings.

The following information is available from the show version command:
IOS version and descriptive information
• Bootstrap ROM version
• Boot ROM version
• Router up time
• Last restart method
• System image file and location
• Router platform
• Configuration register setting

Use the show version command to identify router IOS image and boot
source. To find out the amount of flash memory, issue the show flash
command.
188

Router User Interface Modes
The Cisco command-line interface (CLI) uses a hierarchical structure. This
structure requires entry into different modes to accomplish particular tasks.

Each configuration mode is indicated with a distinctive prompt and allows
only commands that are appropriate for that mode.

As a security feature the Cisco IOS software separates sessions into two
access levels, user EXEC mode and privileged EXEC mode. The privileged
EXEC mode is also known as enable mode.

191

Overview of Router Modes

192

User Mode Commands

194

Privileged Mode Commands

NOTE:
There are
many more
commands
available in
privileged
mode.

195

Specific Configuration Modes

196

CLI Command Modes
All command-line interface (CLI) configuration changes to a Cisco router
are made from the global configuration mode. Other more specific modes
are entered depending upon the configuration change that is required.

Global configuration mode commands are used in a router to apply
configuration statements that affect the system as a whole.

The following command moves the router into global configuration mode

Router#configure terminal (or config t)
Router(config)#

When specific configuration modes are entered, the router prompt changes
to indicate the current configuration mode.

Typing exit from one of these specific configuration modes will return the
router to global configuration mode. Pressing Ctrl-Z returns the router to all
the way back privileged EXEC mode. 197

Configuring a Router’s Name
A router should be given a unique name as one of the
first configuration tasks.

This task is accomplished in global configuration
mode using the following commands:

Router(config)#hostname Tokyo
Tokyo(config)#

As soon as the Enter key is pressed, the prompt
changes from the default host name (Router) to the
newly configured host name (which is Tokyo in the
example above). 198

Setting
the Clock
with Help

199

Message Of The Day (MOTD)
A message-of-the-day (MOTD) banner can be displayed on all
connected terminals.

Enter global configuration mode by using the command config t

Enter the command
banner motd # The message of the day goes here #.

Save changes by issuing the command copy run start

200

Configuring a Console Password
Passwords restrict access to routers.
Passwords should always be configured for virtual terminal
lines and the console line.

Passwords are also used to control access to privileged EXEC
mode so that only authorized users may make changes to the
configuration file.

The following commands are used to set an optional but
recommended password on the console line:

Router(config)#line console 0
Router(config-line)#password <password>
Router(config-line)#login
201

Configuring a Modem Password
If configuring a router via a modem you are most likely
connected to the aux port.

The method for configuring the aux port is very similar to
configuring the console port.

Router(config)#line aux 0

202

Configuring Interfaces
An interface needs an IP Address and a Subnet Mask to be configured.
All interfaces are ―shutdown‖ by default.
The DCE end of a serial interface needs a clock rate.

Router#config t
Router(config)#interface serial 0/1
Router(config-if)#ip address 200.100.50.75 255.255.255.240
Router(config-if)#clock rate 56000 (required for serial DCE only)
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#int f0/0
Router(config-if)#ip address 150.100.50.25 255.255.255.0
Router(config-if)#no shutdown
Router(config-if)#exit
Router(config)#exit
Router#

On older routers, Serial 0/1 would be just Serial 1 and f0/0 would be e0.
s = serial e = Ethernet f = fast Ethernet
203

Configuring a Telnet Password
A password must be set on one or more of the virtual terminal
(VTY) lines for users to gain remote access to the router using
Telnet.

Typically Cisco routers support five VTY lines numbered 0
through 4.

The following commands are used to set the same password
on all of the VTY lines:

Router(config)#line vty 0 4
204

Examining the show Commands
There are many show commands that can be used to examine the contents of files
in the router and for troubleshooting. In both privileged EXEC and user EXEC
modes, the command show ? provides a list of available show commands. The list
is considerably longer in privileged EXEC mode than it is in user EXEC mode.

show interfaces – Displays all the statistics for all the interfaces on the router.
show int s0/1 – Displays statistics for interface Serial 0/1
show controllers serial – Displays information-specific to the interface hardware
show clock – Shows the time set in the router
show hosts – Displays a cached list of host names and addresses
show users – Displays all users who are connected to the router
show history – Displays a history of commands that have been entered
show flash – Displays info about flash memory and what IOS files are stored there
show version – Displays info about the router and the IOS that is running in RAM
show ARP – Displays the ARP table of the router
show start – Displays the saved configuration located in NVRAM
show run – Displays the configuration currently running in RAM
show protocol – Displays the global and interface specific status of any configured
Layer 3 protocols
205

Ethernet Overview
Ethernet is now the dominant LAN technology in the world.

Ethernet is not one technology but a family of LAN
technologies.

All LANs must deal with the basic issue of how individual
stations (nodes) are named, and Ethernet is no exception.

Ethernet specifications support different media, bandwidths,
and other Layer 1 and 2 variations.

However, the basic frame format and addressing scheme is
the same for all varieties of Ethernet. 209

Ethernet and the OSI Model
Ethernet
operates in two
areas of the
OSI model, the
lower half of
the data link
layer, known as
the MAC
sublayer and
the physical
layer
210

Ethernet Technologies
Mapped to the OSI Model

211

Layer 2 Framing
Framing is the Layer 2 encapsulation process.

A frame is the Layer 2 protocol data unit.

The frame format diagram shows different groupings of bits
(fields) that perform other functions.

212

Ethernet and IEEE Frame
Formats are Very Similar

213

3 Common Layer 2 Technologies
Ethernet
Uses CSMA/CD logical bus topology
(information flow is on a linear bus)
physical star or extended star (wired as
a star)

Token Ring
logical ring topology (information flow is
controlled in a ring) and a physical star
topology (in other words, it is wired as a
star)

FDDI
logical ring topology (information flow is
controlled in a ring) and physical dual-
ring topology(wired as a dual-ring)

214

Collision Domains

To move data between one Ethernet station and
another, the data often passes through a repeater.

All other stations in the same collision domain see
traffic that passes through a repeater.

A collision domain is then a shared resource.
Problems originating in one part of the collision
domain will usually impact the entire collision
domain.
215

CSMA/CD Graphic

216

Backoff

After a collision occurs and all stations allow the cable to
become idle (each waits the full interframe spacing), then the
stations that collided must wait an additional and potentially
progressively longer period of time before attempting to
retransmit the collided frame.

The waiting period is intentionally designed to be random so
that two stations do not delay for the same amount of time
before retransmitting, which would result in more collisions.

217

Hierarchical Addressing Using
Variable-Length Subnet Masks

© 2003, Cisco Systems, Inc. All rights reserved. 219

Prefix Length and Network
Mask
Range of Addresses: 192.168.1.64 through 192.168.1.79 Fourth Octet
• Have the first 28 bits in common, which is 64 01000000
represented by a /28 prefix length 65 01000001
• 28 bits in common can also be represented in dotted 66 01000010
decimal as 255.255.255.240 67 01000011
68 01000100
Binary ones in the network mask represent network bits in the 69 01000101
accompanying IP address; binary zeros represent host bits 70 01000110
11000000.10101000.00000001.0100xxxx IP Address 71 01000111
11111111.11111111.11111111.11110000 Network 72 01001000
Mask
73 01001001
In the IP network number that accompanies the network 74 01001010
mask, when the host bits of the IP network number are: 75 01001011
• All binary zeros – that address is the bottom of the 76 01001100
address range 77 01001101
• All binary ones – that address is the top of the 78 01001110
address range 220
79 01001111

Implementing VLSM

221

Range Of Addresses for
VLSM

222

Breakdown Address Space
for Largest Subnet

223

Breakdown Address Space
for Ethernets at Remote Sites

224

Address Space for Serial
Subnets

225

Calculating VLSM: Binary

226

Route Summarization and
Classless Interdomain Routing


What Is Route Summarization?

228

Summarizing Within an Octet

229

Summarizing Addresses in a
VLSM-Designed Network

230

Classless Interdomain Routing

–CIDR is a mechanism developed to alleviate
exhaustion of addresses and reduce routing
table size.
–Block addresses can be summarized into single
entries without regard to the classful boundary of
the network number.
–Summarized blocks are installed in routing
tables.

231

What Is CIDR?

• Addresses are the same as in the route summarization figure, except that
Class B network 172 has been replaced by Class C network 192. 232

Anatomy of an IP Packet
IP packets consist of the data from upper layers plus an IP
header. The IP header consists of the following:

235

Administrative Distance
The administrative distance is an optional parameter that gives a measure
of the reliability of the route. The range of an AD is 0-255 where smaller
numbers are more desireable.

The default administrative distance when using next-hop address is 1, while
the default administrative distance when using the outgoing interface is 0.
You can statically assign an AD as follows:

Router(config)#ip route 172.16.3.0
255.255.255.0 172.16.4.1 130

Sometimes static routes are used for backup purposes. A static route can
be configured on a router that will only be used when the dynamically
learned route has failed. To use a static route in this manner, simply set the
administrative distance higher than that of the dynamic routing protocol
being used. 239

Configuring Default Routes
Default routes are used to route packets with destinations that do
not match any of the other routes in the routing table.

A default route is actually a special static route that uses this format:

ip route 0.0.0.0 0.0.0.0 [next-hop-address | outgoing interface]

This is sometimes referred to as a ―Quad-Zero‖ route.

Example using next hop address:

Router(config)#ip route 0.0.0.0 0.0.0.0 172.16.4.1

Example using the exit interface:

Router(config)#ip route 0.0.0.0 0.0.0.0 s0/0 240

Verifying Static
Route Configuration
After static routes are configured it is important to
verify that they are present in the routing table and
that routing is working as expected.

The command show running-config is used to view
the active configuration in RAM to verify that the static
route was entered correctly.

The show ip route command is used to make sure
that the static route is present in the routing table. 241

Path Determination Graphic

243

Routing Protocol

Router
Switch

Router Router

Router
Router
Switch
What is
an optimal
route ?

244

Routing Protocols
Routing protocols
includes the following:

processes for sharing
route information
allows routers to
communicate with
other routers to update
and maintain the
routing tables

Examples of routing
protocols that support
the IP routed protocol
are:

RIP, IGRP,
OSPF, BGP,
and EIGRP.

245

Routed Protocols
Protocols used at the network layer that transfer data from one host to another across
a router are called routed or routable protocols. The Internet Protocol (IP) and Novell's
Internetwork Packet Exchange (IPX) are examples of routed protocols. Routers use
routing protocols to exchange routing tables and share routing information. In other
words, routing protocols enable routers to route routed protocols.

247

Autonomous System An Autonomous System (AS) is a group of IP networks, which
has a single and clearly defined external routing policy.

EGP
Exterior Gateway
Protocols are used
for routing between
Autonomous Systems

AS 1000 AS 3000

IGP
Interior Gateway Protocols are
used for routing decisions
AS 2000 within an Autonomous System.

249
Fig. 48 IGP and EGP (TI1332EU02TI_0004 The Network Layer, 67)

Interior Gateway Protocol Exterior Gateway Interior Gateway Protocol
(IGP) Protocol (EGP) (IGP)

AS 1000 AS 3000
EGP

EGP IGP
EGP

AS 2000

250
Fig. 49 The use of IGP and EGP protocols (TI1332EU02TI_0004 The Network Layer, 67)

IGP and EGP
An autonomous system is a network or set of networks under
common administrative control, such as the cisco.com domain.

251

Categories of Routing
Protocols
Most routing algorithms can be classified into one of two
categories:

• distance vector
• link-state

The distance vector routing approach determines the direction
(vector) and distance to any link in the internetwork.

The link-state approach, also called shortest path first,
recreates the exact topology of the entire internetwork.
252

Distance Vector
Routing Concepts

253

Distance Vector Routing (DVR)
Destination Distance Routing table contains the addresses
of destinations and the distance
192.16.1.0 1 of the way to this destination.
192.16.5.0 1
192.16.7.0 2

2 Hops

1 Hop 1 Hop

Router A Router B Router C Router D

192.16.1.0 Flow of routing 192.16.7.0
information

192.16.5.0
254

Routing Tables Graphic

255

Distance Vector
Topology Changes

256

Router Metric Components

257

192.16.3.0

192.16.2.0 192.16.6.0

Router A Router B Router C Router D
192.16.4.0
192.16.1.0 192.16.7.0

192.16.5.0

192.16.1.0 0 L 192.16.2.0 0 L 192.16.4.0 0 L 192.16.6.0 0 L
192.16.2.0 0 L 192.16.3.0 0 L 192.16.5.0 0 L 192.16.7.0 0 L
192.16.4.0 0 L 192.16.6.0 0 L

192.16.1.0 0 L 192.16.2.0 0 L 192.16.4.0 0 L 192.16.6.0 0 L
192.16.2.0 0 L 192.16.3.0 0 L 192.16.5.0 0 L 192.16.7.0 0 L
192.16.3.0 1 B 192.16.4.0 0 L 192.16.6.0 0 L 192.16.5.0 1 C
192.16.4.0 1 B 192.16.1.0 1 A 192.16.3.0 1 B 192.16.4.0 1 C
192.16.5.0 1 C 192.16.2.0 1 B

L Locally connected 192.16.6.0 1 C 192.16.7.0 1 D
258


192.16.1.0 0 L 192.16.2.0 0 L 192.16.4.0 0 L 192.16.6.0 0 L

192.16.2.0 0 L 192.16.3.0 0 L 192.16.5.0 0 L 192.16.7.0 0 L

192.16.3.0 1 B 192.16.4.0 0 L 192.16.6.0 0 L 192.16.5.0 1 C

192.16.4.0 1 B 192.16.1.0 1 A 192.16.3.0 1 B 192.16.4.0 1 C

192.16.5.0 2 B 192.16.5.0 1 C 192.16.2.0 1 B 192.16.3.0 2 C

192.16.6.0 2 B 192.16.6.0 1 C 192.16.7.0 1 D 192.16.2.0 2 C

192.16.7.0 2 C 192.16.1.0 2 B

192.16.1.0 0 L 192.16.2.0 0 L 192.16.4.0 0 L 192.16.6.0 0 L

192.16.2.0 0 L 192.16.3.0 0 L 192.16.5.0 0 L 192.16.7.0 0 L
192.16.3.0 1 B 192.16.4.0 0 L 192.16.6.0 0 L 192.16.5.0 1 C

192.16.4.0 1 B 192.16.1.0 1 A 192.16.3.0 1 B 192.16.4.0 1 C

192.16.5.0 2 B 192.16.5.0 1 C 192.16.2.0 1 B 192.16.3.0 2 C

192.16.6.0 2 B 192.16.6.0 1 C 192.16.7.0 1 D 192.16.2.0 2 C

192.16.7.0 3 B 192.16.7.0 2 C 192.16.1.0 2 B 192.16.1.0 3 C

259
Fig. 53 Distribution of routing information with distance vector routing protocol (cont.) (TI1332EU02TI_0004 The Network Layer, 71)

RIPv1
Distance Vector Routing Protocol,
classful

Distribution of Routing Tables via broadcast
to adjacent routers

Fig. 59 Properties of RIPv1 (TI1332EU02TI_0004 The Network Layer, 81)
Only one kind of metric:
Number of Hops

Connections with different
bandwidth can not be weighted

Routing loops can occur
-> bad convergence in case of a failure

Count to infinity problem
(infinity = 16)

Maximum network size is limited
by the number of hops 260

RIP Characteristics

261

RIP-1 permits only a Single Subnet Mask

Port 1
130.24.13.1/24
130.24.13.0/24

RIP-1: 130.24.36.0 RIP-1: 130.24.36.0

130.24.25.0/24 Router A

RIP-1: 130.24.0.0

Port 2 200.14.13.0/24
130.24.36.0/24 200.14.13.2/24

262
Fig. 60 RIP-1 permits only a single subnet mask (TI1332EU02TI_0004 The Network Layer, 83)

Router Configuration
The router command starts a routing process.

The network command is required because it enables the
routing process to determine which interfaces participate in the
sending and receiving of routing updates.

An example of a routing configuration is:

GAD(config)#router rip
GAD(config-router)#network 172.16.0.0

The network numbers are based on the network class
addresses, not subnet addresses or individual host addresses.
263

Configuring RIP Example

264

Verifying RIP Configuration

265

The debug ip rip Command
Most of the RIP
configuration
errors involve an
incorrect network
statement,
discontiguous
subnets, or split
horizons. One
highly effective
command for
finding RIP
update issues is
the debug ip rip
command. The
debug ip rip
command
displays RIP
routing updates
as they are sent
and received. 266

Routing loops
can occur Problem: Routing Loops
when
inconsistent
routing tables
are not
updated due
to slow
convergence
in a changing
network.

267

Problem: Counting to Infinity

268

Solution: Define a Maximum

269

Solution: Split Horizon

270

Route Poisoning
Route poisoning is used by various distance vector protocols in order to
overcome large routing loops and offer explicit information when a subnet
or network is not accessible. This is usually accomplished by setting the
hop count to one more than the maximum.

271

Triggered Updates
New routing tables are sent to neighboring routers on a regular basis.

For example, RIP updates occur every 30 seconds.

However a triggered update is sent immediately in response to some
change in the routing table.

The router that detects a topology change immediately sends an update
message to adjacent routers that, in turn, generate triggered updates
notifying their adjacent neighbors of the change.

When a route fails, an update is sent immediately rather than waiting on the
update timer to expire.

Triggered updates, used in conjunction with route poisoning, ensure that all
routers know of failed routes before any holddown timers can expire.
272

Triggered Updates Graphic

273

Solution: Holddown Timers

274

IGRP
Interior Gateway Routing Protocol (IGRP) is a proprietary
protocol developed by Cisco.

Some of the IGRP key design characteristics emphasize
the following:
• It is a distance vector routing protocol.

• Routing updates are broadcast every 90 seconds.

• Bandwidth, load, delay and reliability are used to
create a composite metric.

275

IGRP Stability Features
IGRP has a number of features that are designed to enhance its stability, such as:
• Holddowns
• Split horizons
• Poison reverse updates

Holddowns
Holddowns are used to prevent regular update messages from inappropriately
reinstating a route that may not be up.

Split horizons
Split horizons are derived from the premise that it is usually not useful to send
information about a route back in the direction from which it came.

Poison reverse updates
Split horizons prevent routing loops between adjacent routers, but poison reverse
updates are necessary to defeat larger routing loops.

Today, IGRP is showing its age, it lacks support for variable length subnet masks
(VLSM). Rather than develop an IGRP version 2 to correct this problem, Cisco has
built upon IGRP's legacy of success with Enhanced IGRP. 276

Configuring IGRP

277

Routing Metrics Graphics

278

Link State Concepts

279

Link State Topology Changes

280

Link State Routing (LSR)
LSP: LSP:
„My links to SPF „My links to R1 and R3 are
R2 and R4 are up.
up― Routing My link to R2 is down.―
Table
Router 1 Router 4

Router 2 Router 3

LSP: „My links to LSP: „My links to
R1 and R3 are up, R2 and R4 are up.―
my link to R4 is down.―

LSP....link state packet
SPF... shortest path first 281

Link State Concerns

282

Link State Routing (LSR)

1
Router A Router C 4

2 2 Router E

1
4
Router B Router D

Link State Database
B-2 A-2 A-1 C-2 C-4
C-1 D-4 D-2 B-4 D-1
E-4 E-1

Router A Router B Router C Router D Router E

A B C D

B C A D D A E C B

D C E E B A
283
E

Link State Routing Features
Link-state algorithms are also known as Dijkstras algorithm or as SPF (shortest path first)
algorithms.

Link-state routing algorithms maintain a complex database of topology information.

The distance vector algorithm are also known as Bellman-Ford algorithms. They have
nonspecific information about distant networks and no knowledge of distant routers.

A link-state routing algorithm maintains full knowledge of distant routers and how they
interconnect. Link-state routing uses:

• Link-state advertisements (LSAs)
A link-state advertisement (LSA) is a small packet of routing information
that is sent between routers.

• Topological database
A topological database is a collection of information gathered from LSAs.

• SPF algorithm
The shortest path first (SPF) algorithm is a calculation performed on the
database resulting in the SPF tree.
284
• Routing tables – A list of the known paths and interfaces.

Link State Routing

285

Comparing Routing Methods

286

OSPF (Open Shortest Path First)
Protocol


OSPF is a Link-State Routing
Protocols
–Link-state (LS) routers recognize much more information
about the network than their distance-vector
counterparts,Consequently LS routers tend to make more accurate
decisions.

–Link-state routers keep track of the following:
• Their neighbours
• All routers within the same area
• Best paths toward a destination

288

Link-State Data Structures

–Neighbor table:
• Also known as the adjacency database
(list of recognized neighbors)

–Topology table:
• Typically referred to as LSDB
(routers and links in the area or network)
• All routers within an area have an identical LSDB

–Routing table:
• Commonly named a forwarding database
(list of best paths to destinations)

289

OSPF vs. RIP
RIP is limited to 15 hops, it converges slowly, and it sometimes chooses
slow routes because it ignores critical factors such as bandwidth in route
determination. OSPF overcomes these limitations and proves to be a
robust and scalable routing protocol suitable for the networks of today.

290

OSPF Terminology
The next several slides explain various OSPF terms -
one per slide.

291

OSPF Term: Link

292

OSPF Term: Link State

293

OSPF Term: Area

294

OSPF Term: Link Cost

295

OSPF Term: Forwarding Database

296

OSPF Term: Adjacencies Database

297

OSPF Terms: DR & BDR

298

Link-State Data Structure:
Network Hierarchy

•Link-state routing requires a hierachical
network structure that is enforced by OSPF.
•This two-level hierarchy consists of the
following:
• Transit area (backbone or area 0)
• Regular areas (nonbackbone areas)

299

Area Terminology

301

LS Data Structures: Adjacency
Database

– Routers discover neighbors by exchanging
hello packets.
– Routers declare neighbors to be up after checking
certain parameters or options in the hello packet.
– Point-to-point WAN links:
• Both neighbors become fully adjacent.
– LAN links:
• Neighbors form an adjacency with the DR and BDR.
• Maintain two-way state with the other routers (DROTHERs).
– Routing updates and topology information are only passed
between adjacent routers.

302

OSPF Adjacencies

Routers build logical adjacencies between each other
using the Hello Protocol. Once an adjacency is formed:
• LS database packets are exchanged to synchronize
each other’s LS databases.
• LSAs are flooded reliably throughout the area or network
using these adjacencies.
303

Open Shortest Path First
Calculation
•Routers find the best paths to destinations by
applying Dijkstra’s SPF algorithm to the link-state
database as follows:
– Every router in an area has the identical
link-state database.
– Each router in the area places itself into
the root of the tree that is built.
– The best path is calculated with respect to the
lowest total cost of links to a specific destination.
– Best routes are put into the forwarding database.
305

OSPF Packet Types

306

OSPF Packet Header Format

307

Establishing Bidirectional
Communication

309

Communication (Cont.)

310

Communication (Cont.)

311

Communication

312

Discovering the Network Routes

313

Discovering the Network Routes

314

Adding the Link-State Entries

315

(Cont.)

316


317

Maintaining Routing Information

• Router A notifies all OSPF DRs on 224.0.0.6
318

(Cont.)

• DR notifies others on 224.0.0.5
319

(Cont.)

320


321

Configuring Basic OSPF:
Single Area
Router(config)#
router ospf process-id

• Turns on one or more OSPF routing processes in
the IOS software.

Router(config-router)#
network address inverse-mask area [area-id]

• Router OSPF subordinate command that defines
the interfaces (by network number) that OSPF
will run on. Each network number must be
defined to a specific area.
322

Configuring OSPF on Internal
Routers of a Single Area

323

Verifying OSPF Operation
Router#
show ip protocols

• Verifies the configured IP routing protocol
processes, parameters and statistics
Router#
show ip route ospf

• Displays all OSPF routes learned by the router
Router#
show ip ospf interface

• Displays the OSPF router ID, area ID and
adjacency information 324

Verifying OSPF Operation
(Cont.)

Router#
show ip ospf

• Displays the OSPF router ID, timers, and statistics

Router#
show ip ospf neighbor [detail]

• Displays information about the OSPF neighbors,
including Designated Router (DR) and Backup
Designated Router (BDR) information on
broadcast networks
325

The show ip route ospf
Command
RouterA# show ip route ospf

Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile,
B - BGP, D - EIGRP, EX - EIGRP external, O - OSPF,
IA - OSPF inter area, E1 - OSPF external type 1,
E2 - OSPF external type 2, E - EGP, i - IS-IS, L1 - IS-IS
level-1, L2 - IS-IS level-2, * - candidate default

Gateway of last resort is not set
10.0.0.0 255.255.255.0 is subnetted, 2 subnets
O 10.2.1.0 [110/10] via 10.64.0.2, 00:00:50, Ethernet0

326

The show ip ospf interface
Command
RouterA# show ip ospf interface e0

Ethernet0 is up, line protocol is up
Internet Address 10.64.0.1/24, Area 0
Process ID 1, Router ID 10.64.0.1, Network Type BROADCAST, Cost: 10
Transmit Delay is 1 sec, State DROTHER, Priority 1
Designated Router (ID) 10.64.0.2, Interface address 10.64.0.2
Backup Designated router (ID) 10.64.0.1, Interface address 10.64.0.1
Timer intervals configured, Hello 10, Dead 40, Wait 40, Retransmit 5
Hello due in 00:00:04
Neighbor Count is 1, Adjacent neighbor count is 1
Adjacent with neighbor 10.64.0.2 (Designated Router)
Suppress hello for 0 neighbor(s)

327

The show ip ospf neighbor
Command
RouterB# show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface
10.64.1.1 1 FULL/BDR 00:00:31 10.64.1.1 Ethernet0
10.2.1.1 1 FULL/- 00:00:38 10.2.1.1 Serial0

328

show ip protocol

show ip route

329

show ip ospf neighbor detail

show ip ospf database

330

OSPF Network Types - 1

331

Point-to-Point Links

• Usually a serial interface running either PPP
or HDLC
• May also be a point-to-point subinterface
running Frame Relay or ATM
• No DR or BDR election required
• OSPF autodetects this interface type
• OSPF packets are sent using multicast 224.0.0.5
332

Multi-access Broadcast Network

• Generally LAN technologies like Ethernet and Token Ring
• DR and BDR selection required
• All neighbor routers form full adjacencies with the DR and
BDR only
• Packets to the DR use 224.0.0.6
• Packets from DR to all other routers use 224.0.0.5 333

Electing the DR and BDR

• Hello packets are exchanged via IP multicast.
• The router with the highest OSPF priority is
selected as the DR.
• Use the OSPF router ID as the tie breaker.
• The DR election is nonpreemptive. 334

Setting Priority for DR Election
Router(config-if)#
ip ospf priority number

• This interface configuration command assigns the
OSPF priority to an interface.
• Different interfaces on a router may be assigned
different values.
• The default priority is 1. The range is from 0 to 255.
• 0 means the router is a DROTHER; it can’t be the
DR or BDR.
335

OSPF Network Types - 2

336

Creation of Adjacencies
RouterA# debug ip ospf adj

Point-to-point interfaces coming up: No election
%LINK-3-UPDOWN: Interface Serial1, changed state to up
OSPF: Interface Serial1 going Up
OSPF: Rcv hello from 192.168.0.11 area 0 from Serial1 10.1.1.2
OSPF: End of hello processing
OSPF: Build router LSA for area 0, router ID 192.168.0.10
OSPF: Rcv DBD from 192.168.0.11 on Serial1 seq 0x20C4 opt 0x2 flag 0x7 len 32
state INIT
OSPF: 2 Way Communication to 192.168.0.11 on Serial1, state 2WAY
OSPF: Send DBD to 192.168.0.11 on Serial1 seq 0x167F opt 0x2 flag 0x7 len 32
OSPF: NBR Negotiation Done. We are the SLAVE
OSPF: Send DBD to 192.168.0.11 on Serial1 seq 0x20C4 opt 0x2 flag 0x2 len 72

337

Creation of Adjacencies (Cont.)
RouterA# debug ip ospf adj

Ethernet interface coming up: Election
OSPF: 2 Way Communication to 192.168.0.10 on Ethernet0, state 2WAY
OSPF: end of Wait on interface Ethernet0
OSPF: DR/BDR election on Ethernet0
OSPF: Elect BDR 192.168.0.12
OSPF: Elect DR 192.168.0.12
DR: 192.168.0.12 (Id) BDR: 192.168.0.12 (Id)
OSPF: Send DBD to 192.168.0.12 on Ethernet0 seq 0x546 opt 0x2 flag 0x7 len 32
<…>
OSPF: DR/BDR election on Ethernet0
OSPF: Elect BDR 192.168.0.11
OSPF: Elect DR 192.168.0.12
DR: 192.168.0.12 (Id) BDR: 192.168.0.11 (Id)

338

Overview
Enhanced Interior Gateway Routing Protocol (EIGRP) is a Cisco-
proprietary routing protocol based on Interior Gateway Routing Protocol
(IGRP).

Unlike IGRP, which is a classful routing protocol, EIGRP supports CIDR
and VLSM.

Compared to IGRP, EIGRP boasts faster convergence times, improved
scalability, and superior handling of routing loops.

Furthermore, EIGRP can replace Novell Routing Information Protocol
(RIP) and AppleTalk Routing Table Maintenance Protocol
(RTMP), serving both IPX and AppleTalk networks with powerful
efficiency.

EIGRP is often described as a hybrid routing protocol, offering the best
340
of distance vector and link-state algorithms.

Comparing EIGRP with IGRP
IGRP and EIGRP are compatible with each other.
EIGRP offers multiprotocol support, but IGRP does not.
EIGRP and IGRP use different metric calculations.
EIGRP scales the metric of IGRP by a factor of 256.
IGRP has a maximum hop count of 255.
EIGRP has a maximum hop count limit of 224.

Enabling dissimilar routing protocols such as OSPF and RIP to
share information requires advanced configuration.
Redistribution, the sharing of routes, is automatic between
IGRP and EIGRP as long as both processes use the same
341
autonomous system (AS) number.

EIGRP & IGRP Metric Calculation

342


343


344

EIGRP Concepts & Terminology
EIGRP routers keep route and topology information readily
available in RAM, so they can react quickly to changes.

Like OSPF, EIGRP saves this information in several tables and
databases.

EIGRP saves routes that are learned in specific ways.

Routes are given a particular status and can be tagged to
provide additional useful information.

EIGRP maintains three tables:
• Neighbor table
• Topology table
• Routing table 345

Neighbor Table
The neighbor table is the most important table in EIGRP.

Each EIGRP router maintains a neighbor table that lists adjacent
routers. This table is comparable to the adjacency database used by
OSPF. There is a neighbor table for each protocol that EIGRP
supports.

When a neighbor sends a hello packet, it advertises a hold time. The
hold time is the amount of time a router treats a neighbor as
reachable and operational. In other words, if a hello packet is not
heard within the hold time, then the hold time expires.

When the hold time expires, the Diffusing Update Algorithm
(DUAL), which is the EIGRP distance vector algorithm, is informed
of the topology change and must recalculate the new topology.
346

Topology Table
The topology table is made up of all the EIGRP routing tables in the
autonomous system.

DUAL takes the information supplied in the neighbor table and the topology
table and calculates the lowest cost routes to each destination. By tracking
this information, EIGRP routers can identify and switch to alternate routes
quickly.

The information that the router learns from the DUAL is used to determine
the successor route, which is the term used to identify the primary or best
route.
A copy is also placed in the topology table.

Every EIGRP router maintains a topology table for each configured network
protocol. All learned routes to a destination are maintained in the topology
table.
347

Routing Table
The EIGRP routing table holds the best routes to a destination. This
information is retrieved from the topology table. Each EIGRP router
maintains a routing table for each network protocol.

A successor is a route selected as the primary route to use to reach a
destination.DUAL identifies this route from the information contained in the
neighbor and topology tables and places it in the routing table.

There can be up to four successor routes for any particular route. These
can be of equal or unequal cost and are identified as the best loop-free
paths to a given destination.

A copy of the successor routes is also placed in the topology table.

A feasible successor (FS) is a backup route.These routes are identified at
the same time the successors are identified, but they are only kept in the
topology table. Multiple feasible successors for a destination can be
348
retained in the topology table although it is not mandatory.

EIGRP Data Structure
Like OSPF, EIGRP relies on different types of packets to maintain its various tables
and establish complex relationships with neighbor routers. The five EIGRP packet
types are:
• Hello
• Acknowledgment
• Update
• Query
• Reply

EIGRP relies on hello packets to discover, verify, and rediscover neighbor routers.

Rediscovery occurs if EIGRP routers do not receive hellos from each other for a
hold time interval but then re-establish communication.

EIGRP routers send hellos at a fixed but configurable interval, called the hello
interval. The default hello interval depends on the bandwidth of the interface.

On IP networks, EIGRP routers send hellos to the multicast IP address 224.0.0.10.
349

Default Hello Intervals
and Hold Times for EIGRP

350

EIGRP Algorithm
The sophisticated DUAL algorithm results in the exceptionally fast
convergence of EIGRP.

Each router constructs a topology table that contains information about how
to route to a destination network.

Each topology table identifies the following:
• The routing protocol or EIGRP
• The lowest cost of the route, which is called Feasible Distance
• The cost of the route as advertised by the neighboring router,
which is called Reported Distance

The Topology heading identifies the preferred primary route, called the
successor route (Successor), and, where identified, the backup
route, called the feasible successor (FS). Note that it is not necessary to
have an identified feasible successor.
351

FS Route Selection Rules

352

Configuring EIGRP

354

Verifying the EIGRP Configuration

To verify the EIGRP configuration a number of show
and debug commands are available.
These commands are shown on the next few slides.

358

show ip eigrp topology

[active | pending | successors]

360

all-links

show ip eigrp traffic

361

Administrative Distances

362

Classful and Classless
Routing Protocols

363

What are ACLs?
ACLs are lists of conditions that are applied to traffic traveling
across a router's interface. These lists tell the router what types
of packets to accept or deny. Acceptance and denial can be
based on specified conditions.

ACLs can be created for all routed network protocols, such as
Internet Protocol (IP) and Internetwork Packet Exchange (IPX).

ACLs can be configured at the router to control access to a
network or subnet.

Some ACL decision points are source and destination
addresses, protocols, and upper-layer port numbers.

ACLs must be defined on a per-protocol, per direction, or per port
365
basis.

Reasons to Create ACLs
The following are some of the primary reasons to create ACLs:

• Limit network traffic and increase network performance.
• Provide traffic flow control.
• Provide a basic level of security for network access.
• Decide which types of traffic are forwarded or blocked at
the router interfaces. For example: Permit e-mail traffic to
be routed, but block all telnet traffic.

Allow an administrator to control what areas a client can access
on a network.

If ACLs are not configured on the router, all packets passing
through the router will be allowed onto all parts of the network.366

ACLs Filter Traffic Graphic

367

How ACLs Filter Traffic

368

One List per Port, per
Destination, per Protocol...

369

Creating ACLs
ACLs are created in the global configuration mode. There are many
different types of ACLs including
standard, extended, IPX, AppleTalk, and others. When configuring
ACLs on a router, each ACL must be uniquely identified by assigning
a number to it. This number identifies the type of access list created
and must fall within the specific range of numbers that is valid for
that type of list.

Since IP is by far the
most popular routed
protocol, addition ACL
numbers have been
added to newer router
IOSs.
Standard IP: 1300-1999
Extended IP: 2000-2699371

The access-list command

372

The ip access-group command

{ in | out }

373

Basic Rules for ACLs
These basic rules should be followed when creating and applying access lists:

• One access list per protocol per direction.
• Standard IP access lists should be applied closest to the destination.
• Extended IP access lists should be applied closest to the source.
• Use the inbound or outbound interface reference as if looking at the port
from inside the router.
• Statements are processed sequentially from the top of list to the bottom
until a match is found, if no match is found then the packet is denied.
• There is an implicit deny at the end of all access lists. This will not appear
in the configuration listing.
• Access list entries should filter in the order from specific to general.
Specific hosts should be denied first, and groups or general filters should
come last.
• Never work with an access list that is actively applied.
• New lines are always added to the end of the access list.
• A no access-list x command will remove the whole list. It is not possible
to selectively add and remove lines with numbered ACLs.
• Outbound filters do not affect traffic originating from the local router.
375

Wildcard Mask Examples
5 Examples follow that demonstrate how a wildcard mask can be
used to permit or deny certain IP addresses, or IP address ranges.

While subnet masks start with binary 1s and end with binary
0s, wildcard masks are the reverse meaning they typically start with
binary 0s and end with binary 1s.

In the examples that follow Cisco has chosen to represent the binary
1s in the wilcard masks with Xs to focus on the specific bits being
shown in each example.

You will see that while subnet masks were ANDed with ip
addresses, wildcard masks are ORed with IP addresses.

.
376

Wildcard Mask Example #1

377


378


379

Wildcard Mask Example #4 - Even IPs

380

Wildcard Mask Example #5 - Odd IP#s

381

The any and host Keywords

382

Verifying ACLs
There are many show commands that will verify the content
and placement of ACLs on the router.

The show ip interface command displays IP interface
information and indicates whether any ACLs are set.

The show access-lists command displays the contents of all
ACLs on the router.

show access-list 1 shows just access-list 1.

The show running-config command will also reveal the
access lists on a router and the interface assignment
information. 383

Standard ACLs
Standard ACLs check the source address of IP packets that are routed.

The comparison will result in either permit or deny access for an entire protocol
suite, based on the network, subnet, and host addresses.

The standard version of the access-list global configuration command is used to
define a standard ACL with a number in the range of 1 to 99 (also from 1300 to
1999 in recent IOS).

If there is no wildcard mask. the default mask is used, which is 0.0.0.0.
(This only works with Standard ACLs and is the same thing as using host.)

The full syntax of the standard ACL command is:

Router(config)#access-list access-list-number
{deny | permit} source [source-wildcard ] [log]

The no form of this command is used to remove a standard ACL. This is the syntax:
384
Router(config)#no access-list access-list-number

Extended ACLs
Extended ACLs are used more often than standard ACLs because they provide a
greater range of control. Extended ACLs check the source and destination packet
addresses as well as being able to check for protocols and port numbers.

The syntax for the extended ACL statement can get very long and often will wrap in
the terminal window.

The wildcards also have the option of using the host or any keywords in the
command.

At the end of the extended ACL statement, additional precision is gained from a field
that specifies the optional Transmission Control Protocol (TCP) or User Datagram
Protocol (UDP) port number.

Logical operations may be specified such as, equal (eq), not equal (neq), greater
than (gt), and less than (lt), that the extended ACL will perform on specific protocols.

Extended ACLs use an access-list-number in the range 100 to 199 (also from 2000
to 2699 in recent IOS). 385

Extended ACL Syntax

386

Extended ACL Example
This extended ACL will allow people in network 200.100.50.0
to surfing the internet, but not allow any other protocols like
email, ftp, etc.

access-list 101 permit tcp 200.100.50.0 0.0.0.255 any eq 80
or
access-list 101 permit tcp 200.100.50.0 0.0.0.255 any eq www
or
access-list 101 permit tcp 200.100.50.0 0.0.0.255 any eq http

NOTE: Just like all Standard ACLs end with an implicit "deny
any", all Extended ACLs end with an implicit "deny ip any any"
which means deny the entire internet from anywhere to
anywhere. 388

ip access-group
The ip access-group command links an existing standard or
extended ACL to an interface.

Remember that only one ACL per interface, per direction, per
protocol is allowed.

The format of the command is:

Router(config-if)#ip access-group
access-list-number {in | out}

389

Named ACLs
IP named ACLs were introduced in Cisco IOS Software Release
11.2, allowing standard and extended ACLs to be given names instead of
numbers.

The advantages that a named access list provides are:
• Intuitively identify an ACL using an alphanumeric name.
• Eliminate the limit of 798 simple and 799 extended ACLs
• Named ACLs provide the ability to modify ACLs without deleting
them completely and then reconfiguring them.

Named ACLs are not compatible with Cisco IOS releases prior to Release
11.2.

The same name may not be used for multiple ACLs.

390

Named ACL Example

391

Placing ACLs
The general rule is to put the extended ACLs as close as possible to the
source of the traffic denied. Standard ACLs do not specify destination
addresses, so they should be placed as close to the destination as
possible. For example, in the graphic a standard ACL should be placed on
Fa0/0 of Router D to prevent traffic from Router A.

392

Permitting a Single Host
Router(config)# access-list 1 permit 200.100.50.23 0.0.0.0
or
Router(config)# access-list 1 permit host 200.100.50.23
or
Router(config)# access-list 1 permit 200.100.50.23

(The implicit ―deny any‖ ensures that everyone else is denied.)

Router(config)# int e0
Router(config-if)# ip access-group 1 in
or
Router(config-if)# ip access-group 1 out
394

Denying a Single Host
Router(config)# access-list 1 deny 200.100.50.23 0.0.0.0
or
Router(config)# access-list 1 deny host 200.100.50.23
Router(config)# access-list 1 permit any

(The implicit ―deny any‖ is still present, but totally irrelevant.)

or
395

Permitting a Single Network
Class C
or
Class B
or
Class A


or
396

Denying a Single Network
Class C
or
Class B
or
Class A

397

Permitting a Class C Subnet
Network Address/Subnet Mask: 200.100.50.0/28
Desired Subnet: 3rd

Process:
32-28=4 2^4 = 16
1st Usable Subnet address range it 200.100.50.16-31
2nd Usable Subnet address range it 200.100.50.32-47
3rd Usable Subnet address range it 200.100.50.48-63

Subnet Mask is 255.255.255.240 Inverse Mask is 0.0.0.15
or subtract 200.100.50.48 from 200.100.50.63 to get 0.0.0.15


398

Denying a Class C Subnet
Undesired Subnet: 2nd

Process:
32-27=5 2^5=32
1st Usable Subnet address range it 192.68.72.32-63
2nd Usable Subnet address range it 192.68.72.64-95



399

Permitting a Class B Subnet
Desired Subnet: 129th

Process:
Since exactly 8 bits are borrowed the 3rd octet will denote the
subnet number.
129th Usable Subnet address range it 150.75.129.0-255



400

Denying a Class B Subnet
Undesired Subnet: 50th

Process:
32-22=10 (more than 1 octet) 10-8=2 2^2=4
1st Usable Subnet address range it 160.88.4.0-160.88.7.255
2nd Usable Subnet address range it 160.88.8.0-160.88.11.255

50 * 4 = 200 50th subnet is 160.88.200.0-160.88.203.255


401

Permitting a Class A Subnet
Desired Subnet: 13th

Process:
32-12=20 20-16=4 2^4=16
1st Usable Subnet address range is 111.16.0.0-111.31.255.255
13*16=208
13th Usable Subnet address range is 111.208.0.0-111.223.255.255



402

Denying a Class A Subnet
Undesired Subnet: 500th

Process:
Since exactly 16 bits were borrowed the 2nd and 3rd octet will
denote the subnet.

1st Usable Subnet address range is 40.0.1.0-40.0.1.255

Router(config)# access-list 1 deny 40.1.244.0 0 0.0.0.255
403

Permit 200.100.50.24-100 Plan A
access-list 1 permit host 200.100.50.24
access-list 1 permit host 200.100.50.26 This
access-list 1 permit host 200.100.50.27 would
access-list 1 permit host 200.100.50.28 get very
: : : : : : : : tedious!
405

Permit 200.100.50.24-100 Plan B
access-list 1 permit 200.100.50.24 0.0.0.7 (24-31)




access-list 1 permit host 200.100.50.100 (100)


406

Permit 200.100.50.16-127 Plan A




407

Permit 200.100.50.16-127 Plan B
access-list 1 deny 200.100.50.0 0.0.0.15 (0-15)


First we make sure that addresses 0-15 are denied.

Then we can permit any address in the range 0-127.

Since only the first matching statement in an ACL is applied an
address in the range of 0-15 will be denied by the first statement
before it has a chance to be permitted by the second.

408

Permit 200.100.50.1,5,13,29,42,77

Sometimes a group of addresses has no pattern and the best
way to deal with them is individually.

409

Permit Source Network
access-list 101 permit ip 200.100.50.0 0.0.0.255
0.0.0.0 255.255.255.255
or

access-list 101 permit ip 200.100.50.0 0.0.0.255 any

Implicit deny ip any any

411

Deny Source Network
access-list 101 deny ip 200.100.50.0 0.0.0.255
0.0.0.0 255.255.255.255
0.0.0.0 255.255.255.255
or

access-list 101 deny ip 200.100.50.0 0.0.0.255 any
access-list 101 permit ip any any

Implicit deny ip any any is present but irrelevant.
412

Permit Destination Network
200.100.50.0 0.0.0.255
or

access-list 101 permit ip any 200.100.50.0 0.0.0.255


413

Deny Destination Network
200.100.50.0 0.0.0.255
0.0.0.0 255.255.255.255
or

access-list 101 deny ip any 200.100.50.0 0.0.0.255

Implicit deny ip any any is present but irrelevant.
414

Permit one Source Network to
another Destination Network
Assume the only traffic you want is traffic from network
200.100.50.0 to network 150.75.0.0

150.75.0.0 0.0.255.255

To allow 2 way traffic between the networks add this statement:

415
200.100.50.0 0.0.0.255

Deny one Source Network to
another Destination Network
Assume you want to allow all traffic EXCEPT from network
200.100.50.0 to network 150.75.0.0

150.75.0.0 0.0.255.255

To deny 2 way traffic between the networks add this statement:

416
200.100.50.0 0.0.0.255

Deny FTP
Assume you do not want anyone FTPing on the network.

access-list 101 deny tcp any any eq 21

or

access-list 101 deny tcp any any eq ftp
417

Deny Telnet
Assume you do not want anyone telnetting on the network.


or

access-list 101 deny tcp any any eq telnet
418

Deny Web Surfing
Assume you do not want anyone surfing the internet.


or

access-list 101 deny tcp any any eq www

You can also use http instead of www. 419

Complicated Example #1
Suppose you have the following conditions:
 No one from Network 200.100.50.0 is allowed to FTP anywhere
 Only hosts from network 150.75.0.0 may telnet to network 50.0.0.0
 Subnetwork 100.100.100.0/24 is not allowed to surf the internet

access-list 101 deny tcp 200.100.50.0 0.0.0.255 any eq 21

access-list 101 permit tcp 150.75.0.0 0.0.255.255 50.0.0.0
0.255.255.255 eq 23


access-list 101 deny tcp 100.100.100.0 0.0.0.255 any eq 80

420

Complicated Example #2
Suppose you are the admin of network 200.100.50.0. You want to permit
Email only between your network and network 150.75.0.0. You wish to place
no restriction on other protocols like web surfing, ftp, telnet, etc.
 Email server send/receive Protocol: SMTP, port 25
 User Check Email Protocol: POP3, port 110
This example assumes the your Email server is at addresses 200.100.50.25

access-list 101 permit tcp 200.100.50.0 0.0.0.255 150.75.0.0
0.0.255.255 eq 25
access-list 101 permit tcp 150.75.0.0 0.0.255.255
200.100.50.0 0.0.0.255 eq 25
access-list 101 permit tcp 200.100.50.0 0.0.0.255
200.100.50.0 0.0.0.255 eq 110
access-list 101 deny tcp any any smtp
access-list 101 deny tcp any any pop3
access-list 101 permit ip any any 421

NAT
Network Address Translator

422
Fig. 3 NAT (TI1332EU02TI_0003 New Address Concepts, 7)

New addressing concepts

Problems with IPv4
Shortage of IPv4 addresses
Allocation of the last IPv4 addresses is forecasted for the year 2005
Address classes were replaced by usage of CIDR, but this is not sufficient

Short term solution
NAT: Network Address Translator

Long term solution
IPv6 = IPng (IP next generation)
Provides an extended address range

423
Fig. 2 Address shortage and possible solutions (TI1332EU02TI_0003 New Address Concepts, 5)


NAT
Translates between local addresses and public ones
Many private hosts share few global addresses

Private Network Public Network
Uses private address range Uses public addresses
(local addresses)
Local addresses may not Public addresses are
be used externally globally unique
424
Fig. 4 How does NAT work? (TI1332EU02TI_0003 New Address Concepts, 9)

realm with realm with
private addresses public addresses

translate reserve
To be pool
translated

map
NAT

exclude exclude

NAT Router

425
Fig. 5 Translation mechanism (TI1332EU02TI_0003 New Address Concepts, 9)

free
NAT
Pool

A timeout value (default 15 min) instructs NAT
how long to keep an association in an idle state before
returning the external IP address to the free NAT pool.
426
Fig. 8 How does NAT know when to return the public IP address to the pool? (TI1332EU02TI_0003 New Address Concepts, 15)

NAT Addressing Terms
• Inside Local
– The term ―inside‖ refers to an address used for a host
inside an enterprise. It is the actual IP address
assigned to a host in the private enterprise network.

• Inside Global
– NAT uses an inside global address to represent the
inside host as the packet is sent through the outside
network, typically the Internet.
– A NAT router changes the source IP address of a
packet sent by an inside host from an inside local
address to an inside global address as the packet goes
from the inside to the outside network. 427

• Outside Global
– The term ―outside‖ refers to an address used for a
host outside an enterprise, the Internet.
– An outside global is the actual IP address assigned to
a host that resides in the outside network, typically
the Internet.
• Outside Local
– NAT uses an outside local address to represent the
outside host as the packet is sent through the private
enterprise network.
– A NAT router changes a packet’s destination IP
address, sent from an outside global address to an
inside host, as the packet goes from the outside to the
inside network.
428

WAN

Router

Router A with NAT
Router Router Router B
SA = 193.50.30.4

DA = 192.50.20.5

SA = 10.47.10.10
Router Router
DA = 192.50.20.5 Net B
192.50.20.0
LAN LAN
Net A
10.0.0.0
S
I
E
M
N
X
D
O
R
F S
I
E
M
N
X
D
O
R
F
10.47.10.10 192.50.20.5

429
Fig. 7 An example for NAT (TI1332EU02TI_0003 New Address Concepts, 13)

WAN
NAT with Router
WAN interface:
138.76.28.4 S
I
E
M
N
X
D
O
R
F
138.76.29.7
Router

SA = 138.76.28.4 SA = 138.76.29.7
DA =138.76.29.7 DA = 138.76.28.4

Router
SA = 10.0.0.10 SA = 138.76.29.7
DA = 138.76.29.7 DA = 10.0.0.10

Net A
10.0.0.0/8
S
I
E
M
N
X
D
O
R
F
10.0.0.10

430
Fig. 11 An example for NAPT (TI1332EU02TI_0003 New Address Concepts, 21)

Types Of NAT
• There are different types of NAT that can
be used, which are
– Static NAT
– Dynamic NAT
– Overloading NAT with PAT (NAPT)

431

Static NAT
• With static NAT, the NAT router simply
configures a one-to-one mapping between
the private address and the registered
address that is used on its behalf.

432

Dynamic NAT
• Like static NAT, the NAT router creates a
one-to-one mapping between an inside
local and inside global address and
changes the IP addresses in packets as
they exit and enter the inside network.

• However, the mapping of an inside local
address to an inside global address
happens dynamically.
434

Dynamic NAT

• Dynamic NAT sets up a pool of possible
inside global addresses and defines
criteria for the set of inside local IP
addresses whose traffic should be
translated with NAT.

• The dynamic entry in the NAT table stays
in there as long as traffic flows
occasionally. 435

PAT
Port Address Translator

436
Fig. 9 NAPT (TI1332EU02TI_0003 New Address Concepts, 17)

WAN
NAPT with Router
WAN interface:
138.76.28.4 S
I
E
M
N
X
D
O
R
F
138.76.29.7
Router

SA = 138.76.28.4, sport = 1024 SA = 138.76.29.7, spor t= 23
DA =138.76.29.7, dpor t= 23 DA = 138.76.28.4, dport = 1024

Router
SA = 10.0.0.10, sport = 3017 SA = 138.76.29.7, spor t= 23
DA = 138.76.29.7, dpor t= 23 DA = 10.0.0.10, dport = 3017

Net A
10.0.0.0/8
S
I
E
M
N
X
D
O
R
F
10.0.0.10

437

PAT with e.g. a single public IP address

single public
IP address
private IP network
WAN
(e.g. SOHO)

pool of TU port numbers

local IP @,
registered IP @,
local TU port # mapping
assigned TU port #

438
TU....TCP/UDP

NAT&PAT
Network Address Translation &
Port Address Transation

439
Fig. 3 NAT (TI1332EU02TI_0003 New Address Concepts, 7)

New addressing concepts

Problems with IPv4
Shortage of IPv4 addresses
Allocation of the last IPv4 addresses is forecasted for the year 2006
Address classes were replaced by usage of CIDR, but this is not sufficient

Short term solution

Long term solution
IPv6 = IPng (IP next generation)
Provides an extended address range

440


NAT
Translates between local addresses and public ones
Many private hosts share few global addresses

Private Network Public Network
Uses private address range Uses public addresses
(local addresses)
Local addresses may not Public addresses are
be used externally globally unique
441
Fig. 4 How does NAT work? (TI1332EU02TI_0003 New Address Concepts, 9)

private addresses public addresses

translate reserve
To be pool
translated

map
NAT

exclude exclude

NAT Router

442
Fig. 5 Translation mechanism (TI1332EU02TI_0003 New Address Concepts, 9)

free
NAT
Pool

A timeout value (default 15 min) instructs NAT
how long to keep an association in an idle state before
returning the external IP address to the free NAT pool.
443
Fig. 8 How does NAT know when to return the public IP address to the pool? (TI1332EU02TI_0003 New Address Concepts, 15)

• Inside Local ―Private address‖
– The term ―inside‖ refers to an address used for a host
inside an enterprise. It is the actual IP address
assigned to a host in the private enterprise network.

• Inside Global ―Public address‖
– NAT uses an inside global address to represent the
inside host as the packet is sent through the outside
network, typically the WAN.
– A NAT router changes the source IP address of a
packet sent by an inside host from an inside local
address to an inside global address as the packet goes
from the inside to the outside network. 444

WAN

Router

Router A with NAT
Router Router Router B
SA = 193.50.30.4

DA = 192.50.20.5

SA = 10.47.10.10
Router Router
DA = 192.50.20.5 Net B
192.50.20.0
LAN LAN
Net A
10.0.0.0
S
I
E
M
N
X
D
O
R
F S
I
E
M
N
X
D
O
R
F
10.47.10.10 192.50.20.5

445
Fig. 7 An example for NAT (TI1332EU02TI_0003 New Address Concepts, 13)

WAN
NAT with Router
WAN interface:
138.76.28.4 S
I
E
M
N
X
D
O
R
F
138.76.29.7
Router

SA = 138.76.28.4 SA = 138.76.29.7
DA =138.76.29.7 DA = 138.76.28.4

Router
SA = 10.0.0.10 SA = 138.76.29.7
DA = 138.76.29.7 DA = 10.0.0.10

Net A
10.0.0.0/8
S
I
E
M
N
X
D
O
R
F
10.0.0.10

446

Types Of NAT
• There are different types of NAT that can
be used, which are
– Static NAT
– Dynamic NAT
– Overloading NAT with PAT (NAT Over PAT)

447

Static NAT
• With static NAT, the NAT router simply
configures a one-to-one mapping between
the private address and the registered
address that is used on its behalf.

448

Static NAT Configuration

• To form NAT table
Router(config)#IP Nat inside source static [inside local
source IP address] [inside global source IP address]

• Assign NAT to an Interface
Router(config)#Interface [Serial x/y]
Router(config-if)#IP NAT [Inside]

• See Example

450

Dynamic NAT
• Like static NAT, the NAT router creates a
one-to-one mapping between an inside
local and inside global address and
changes the IP addresses in packets as
they exit and enter the inside network.

• However, the mapping of an inside local
address to an inside global address
happens dynamically.
451

Dynamic NAT

• Dynamic NAT sets up a pool of possible inside
global addresses and defines criteria for the
set of inside local IP addresses whose traffic
should be translated with NAT.

• The dynamic entry in the NAT table stays in
there as long as traffic flows occasionally.

• If a new packet arrives, and it needs a NAT
entry, but all the pooled IP addresses are in 452
use, the router simply discards the packet.

Dynamic NAT Configuration

• Specify inside addresses to be translated
Router(config)#IP Nat inside source list [standard Access
List number] pool [NAT Pool Name]

• Specify NAT pool
Router(config)#IP Nat pool [NAT Pool Name] [First inside
global address] [Last inside global address] netmask
[subnet mask]

• Assign NAT to an Interface

• See Example
453

PAT
Port Address Translator

454

WAN
NAPT with Router
WAN interface:
138.76.28.4 S
I
E
M
N
X
D
O
R
F
138.76.29.7
Router

SA = 138.76.28.4, sport = 1024 SA = 138.76.29.7, spor t= 23
DA =138.76.29.7, dpor t= 23 DA = 138.76.28.4, dport = 1024

Router
SA = 10.0.0.10, sport = 3017 SA = 138.76.29.7, spor t= 23
DA = 138.76.29.7, dpor t= 23 DA = 10.0.0.10, dport = 3017

Net A
10.0.0.0/8
S
I
E
M
N
X
D
O
R
F
10.0.0.10

455

PAT with e.g. a single public IP address

single public
IP address
private IP network
WAN
(e.g. SOHO)

pool of TU port numbers

local IP @,
registered IP @,
local TU port # mapping
assigned TU port #

456
TU....TCP/UDP

PAT Configuration

• Specify inside addresses to be translated
Router(config)#IP Nat inside source list [standard Access
List number] pool [NAT Pool Name] overload

• Specify PAT pool
Router(config)#IP Nat pool [NAT Pool Name] [First inside
global address] [Last inside global address] netmask
[subnet mask]

• Assign PAT to an Interface

• See Example
457

Ethernet Access with Hubs

459

Ethernet Access with Bridges

460

Ethernet Access with Switches

461

Full Duplex Transmitting
Full-duplex Ethernet allows the transmission of a packet and the reception of a
different packet at the same time.
This simultaneous transmission and reception requires the use of two pairs of wires
in the cable and a switched connection between each node. This connection is
considered point-to-point and is collision free.
The full-duplex Ethernet switch takes advantage of the two pairs of wires in the
cable by creating a direct connection between the transmit (TX) at one end of the
circuit and the receive (RX) at the other end.
Ethernet usually can only use 50%-60% of the available 10 Mbps of bandwidth
because of collisions and latency. Full-duplex Ethernet offers 100% of the
bandwidth in both directions. This produces a potential 20 Mbps throughput.

463

Collision Domains

465

Segmentation with Bridges

466

Segmentation with Routers

467

Segmentation with Switches

468

Basic Operations of a Switch
Switching is a technology that decreases congestion in Ethernet, Token
Ring, and FDDI LANs. Switching accomplishes this by reducing traffic and
increasing bandwidth. LAN switches are often used to replace shared hubs
and are designed to work with existing cable infrastructures.
Switching equipment performs the following two basic operations:
• Switching data frames
• Maintaining switching operations

469

Switching Methods
1. Store-and-Forward
The entire frame is received before any forwarding takes place. Filters are
applied before the frame is forwarded. Most reliable and also most latency
especially when frames are large.

2. Cut-Through
The frame is forwarded through the switch before the entire frame is
received. At a minimum the frame destination address must be read before
the frame can be forwarded. This mode decreases the latency of the
transmission, but also reduces error detection.

3. Fragment-Free
Fragment-free switching filters out collision fragments before forwarding
begins. Collision fragments are the majority of packet errors. In a properly
functioning network, collision fragments must be smaller than 64 bytes.
Anything > 64 bytes is a valid packet and is usually received without error.

470

Frame Transmission Modes

471

Benefits of Switching

472

How Switches and Bridges
Learn Addresses
Bridges and switches learn in the following ways:

• Reading the source MAC address of each
received frame or datagram

• Recording the port on which the MAC address
was received.

In this way, the bridge or switch learns which addresses
belong to the devices connected to each port.
473

CAM
Content Addressable Memory
CAM is used in switch applications:

• To take out and process the address information from
incoming data packets

• To compare the destination address with a table of
addresses stored within it

The CAM stores host MAC addresses and associated port
numbers. The CAM compares the received destination MAC
address against the CAM table contents. If the comparison
yields a match, the port is provided, and switching control
474
forwards the packet to the correct port and address.

Shared vs. Dedicates Bandwidth
If a hub is used, bandwidth is shared. If a switch is used, then bandwidth is
dedicated. If a workstation or server is directly connected to a switch port, then the
full bandwidth of the connection to the switch is available to the connected
computer. If a hub is connected to a switch port, bandwidth is shared between all
devices connected to the hub.

475

Microsegmentation of a Network

476

Microsegmentation

477

3 Methods of Communication

478

Switches & Broadcast Domains
When two switches are connected, the broadcast domain is increased.
The overall result is a reduction in available bandwidth. This happens because all
devices in the broadcast domain must receive and process the broadcast frame.
Routers are Layer 3 devices. Routers do not propagate broadcasts. Routers are
used to segment both collision and broadcast domains.

479

Broadcast Domain

480

Overview
To design reliable, manageable, and scalable networks, a network
designer must realize that each of the major components of a
network has distinct design requirements.

Good network design will improve performance and also reduce the
difficulties associated with network growth and evolution.

The design of larger LANs includes identifying the following:
• An access layer that connects end users into the LAN
• A distribution layer that provides policy-based connectivity
between end-user LANs
• A core layer that provides the fastest connection between
the distribution points

Each of these LAN design layers requires switches that are best
suited for specific tasks. 482

The Access Layer
The access layer is the entry point for user workstations and servers to
the network. In a campus LAN the device used at the access layer can
be a switch or a hub.

Access layer functions also include MAC layer filtering and
microsegmentation. Layer 2 switches are used in the access layer.

483

Access Layer Switches
Access layer switches operate at Layer 2 of the OSI model

The main purpose of an access layer switch is to allow end
users into the network.

An access layer switch should provide this functionality with
low cost and high port density.

The following Cisco switches are commonly used at the
access layer:
• Catalyst 1900 series
484

The Distribution Layer
The distribution layer of the network is between the access and core layers.
Networks are segmented into broadcast domains by this layer. Policies can be
applied and access control lists can filter packets.

The distribution layer isolates network problems to the workgroups in which they
occur. The distribution layer also prevents these problems from affecting the core
layer. Switches in this layer operate at Layer 2 and Layer 3.

485

Distribution Layer Switches
The distribution layer switch must have high performance.

The distribution layer switch is a point at which a broadcast domain is
delineated. It combines VLAN traffic and is a focal point for policy
decisions about traffic flow.

For these reasons distribution layer switches operate at both Layer 2
and Layer 3 of the OSI model.

Switches in this layer are referred to as multilayer switches. These
multilayer switches combine the functions of a router and a switch in
one device.

The following Cisco switches are suitable for the distribution layer:
• Catalyst 2926G
• Catalyst 5000 family
• Catalyst 6000 family 486

The Core Layer
The core layer is a high-speed switching backbone.

This layer of the network design should not perform any packet manipulation.
Packet manipulation, such as access list filtering, would slow down the process.

Providing a core infrastructure with redundant alternate paths gives stability to the
network in the event of a single device failure.

The core can be designed to use Layer 2 or Layer 3 switching. Asynchronous
Transfer Mode (ATM) or Ethernet switches can be used.

487

Core Layer Switches
The switches in this layer can make use of a number of Layer 2
technologies. Provided that the distance between the core layer
switches is not too great, the switches can use Ethernet technology.

In a network design, the core layer can be a routed, or Layer 3, core.
Core layer switches are designed to provide efficient Layer 3
functionality when needed.

Factors such as need, cost, and performance should be considered
before a choice is made.

The following Cisco switches are suitable for the core layer:
• IGX 8400 series
• Lightstream 1010 488

Physical Startup of the Catalyst Switch
Switches are dedicated, specialized
computers, which contain a CPU, RAM, and
an operating system.

Switches usually have several ports for the
purpose of connecting hosts, as well as
specialized ports for the purpose of
management.

A switch can be managed by connecting to
the console port to view and make changes
to the configuration.

Switches typically have no power switch to
turn them on and off. They simply connect or
disconnect from a power source.

Several switches from the Cisco Catalyst
2950 series are shown in graphic to the right. 490

Switch LED Indicators
The front panel of a switch has several lights to help monitor system
activity and performance. These lights are called light-emitting diodes
(LEDs). The switch has the following LEDs:

• System LED
• Remote Power Supply (RPS) LED
• Port Mode LED
• Port Status LEDs

The System LED shows whether the system is receiving power and
functioning correctly.

The RPS LED indicates whether or not the remote power supply is in use.

The Mode LEDs indicate the current state of the Mode button.

The Port Status LEDs have different meanings, depending on the current
value of the Mode LED. 491

Verifying Port LEDs During Switch POST
Once the power cable is connected, the switch initiates a
series of tests called the power-on self test (POST).

POST runs automatically to verify that the switch functions
correctly.

The System LED indicates the success or failure of POST.

492

Connecting a Switch to a Computer

493

Examining Help in the Switch CLI
The command-line interface (CLI) for Cisco switches is very
similar to the CLI for Cisco routers.

The help command is issued by entering a question mark (?).

When this command is entered at the system prompt, a list of
commands available for the current command mode is
displayed.

The help command is very flexible and essentially functions
the same way it does in a router CLI.

This form of help is called command syntax help, because it
provides applicable keywords or arguments based on a partial
command. 494

Switch Command Modes
Switches have several command modes.

The default mode is User EXEC mode, which ends in a
greater-than character (>).

The commands available in User EXEC mode are limited to
those that change terminal settings, perform basic tests, and
display system information.

The enable command is used to change from User EXEC
mode to Privileged EXEC mode, which ends in a pound-sign
character (#).

The configure command allows other command modes to be
accessed. 495

Show Commands in User-Exec Mode

496

Setting Switch Hostname
Setting Passwords on Lines

497

Overview
Redundancy in a network is extremely important because
redundancy allows networks to be fault tolerant.

Redundant topologies based on switches and bridges are
susceptible to broadcast storms, multiple frame
transmissions, and MAC address database instability.

Therefore network redundancy requires careful planning
and monitoring to function properly.

The Spanning-Tree Protocol is used in switched networks
to create a loop free logical topology from a physical
topology that has loops.
499

Redundant Switched Topologies
Networks with redundant paths and devices allow for more network uptime.
In the graphic, if Switch A fails, traffic can still flow from Segment 2 to
Segment 1 and to the router through Switch B. If port 1 fails on Switch A then
traffic can still flow through port 1 on Switch B.
Switches learn the MAC addresses of devices on their ports so that data can
be properly forwarded to the destination. Switches will flood frames for
unknown destinations until they learn the MAC addresses of the devices.
A redundant switched topology may cause broadcast storms, multiple frame
copies, and MAC address table instability problems.

500

Broadcast Storms
Broadcasts and multicasts can cause problems in a switched network.
Multicasts are treated as broadcasts by the switches.

Broadcasts and multicasts frames are flooded out all ports, except the one on
which the frame was received.

The switches continue to propagate broadcast traffic over and over. This is
called a broadcast storm. This will continue until one of the switches is
disconnected. The network will appear to be down or extremely slow.

501

Multiple Frame Transmissions
In a redundant switched network it is possible for an end device to receive
multiple frames. Assume that the MAC address of Router Y has been timed
out by both switches. Also assume that Host X still has the MAC address of
Router Y in its ARP cache and sends a unicast frame to Router Y. The router
receives the frame because it is on the same segment as Host X. Switch A
does not have the MAC address of the Router Y and will therefore flood the
frame out its ports. Switch B also does not know which port Router Y is on.
Switch B then floods the frame it received causing Router Y to receive
multiple copies of the same frame. This is a cause of unnecessary processing
in all devices.

502

MAC Database Instability
A switch can incorrectly learn that a MAC address is on one port, when it is
actually on a different port. In this example the MAC address of Router Y is
not in the MAC address table of either switch. Host X sends a frame directed
to Router Y. Switches A & B learn the MAC address of Host X on port 0. The
frame to Router Y is flooded on port 1 of both switches. Switches A and B see
this information on port 1 and incorrectly learn the MAC address of Host X on
port 1. When Router Y sends a frame to Host X, Switch A and Switch B will
also receive the frame and will send it out port 1. This is unnecessary, but the
switches have incorrectly learned that Host X is on port 1.

503

Using Bridging Loops
for Redundancy

504

Logical Loop Free Topology
Created with STP

505

NOTE:
Don’t confuse Spanning Tree Protocol
(STP) with Shielded Twisted Pair (STP).

506

Spanning Tree Protocol - 1
Ethernet bridges
and switches can
implement the
IEEE 802.1D
Spanning-Tree
Protocol and use
the spanning-tree
algorithm to
construct a loop
free shortest path
network.

Shortest path is
based on
cumulative link
costs.
Link costs are
based on the
speed of the link. 507

Spanning Tree Protocol - 2
The Spanning-Tree Protocol
establishes a root node, called the
root bridge/switch.

The Spanning-Tree Protocol
constructs a topology that has one
path for reaching every network
node. The resulting tree originates
from the root bridge/switch.

The Spanning-Tree Protocol requires
network devices to exchange
messages to detect bridging loops.
Links that will cause a loop are put
into a blocking state.

The message that a switch
sends, allowing the formation of a
loop free logical topology, is called a
Bridge Protocol Data Unit (BPDU).
508

Selecting the Root Bridge
The first decision that all switches in the network make, is to identify
the root bridge. The position of the root bridge in a network will affect
the traffic flow.

When a switch is turned on, the spanning-tree algorithm is used to
identify the root bridge. BPDUs are sent out with the Bridge ID (BID).

The BID consists of a bridge priority that defaults to 32768 and the
switch base MAC address.

When a switch first starts up, it assumes it is the root switch and
sends BPDUs. These BPDUs contain the switch MAC address in both
the root and sender BID. As a switch receives a BPDU with a lower
root BID it replaces that in the BPDUs that are sent out. All bridges
see these and decide that the bridge with the smallest BID value will
be the root bridge.

A network administrator may want to influence the decision by setting
509
the switch priority to a smaller value than the default.

BDPUs
BPDUs contain enough information so that all switches can do
the following:
• Select a single switch that will act as the root of the
spanning tree
• Calculate the shortest path from itself to the root switch
• Designate one of the switches as the closest one to the
root, for each LAN segment. This bridge is called the
―designated switch‖. The designated switch handles all
communication from that LAN towards the root bridge.
• Each non-root switch choose one of its ports as its root
port, this is the interface that gives the best path to the
root switch.
• Select ports that are part of the spanning tree, the
designated ports. Non-designated ports are blocked.

510

Spanning Tree Operation
When the network has stabilized, it has converged and there is one spanning
tree per network. As a result, for every switched network the following
elements exist:
• One root bridge per network
• One root port per non root bridge
• One designated port per segment
• Unused, non-designated ports
Root ports and designated ports are used for forwarding (F) data traffic.
Non-designated ports discard data traffic.
Non-designated ports are called blocking (B) or discarding ports.

511

Spanning Tree Port States

512

Spanning Tree Recalculation
A switched internetwork has converged when all the switch and
bridge ports are in either the forwarding or blocked state.

Forwarding ports send and receive data traffic and BPDUs.

Blocked ports will only receive BPDUs.

When the network topology changes, switches and bridges
recompute the Spanning Tree and cause a disruption of user
traffic.

Convergence on a new spanning-tree topology using the IEEE
802.1D standard can take up to 50 seconds.

This convergence is made up of the max-age of 20 seconds, plus
the listening forward delay of 15 seconds, and the learning forward
delay of 15 seconds. 513

Rapid STP Designations

514

VLANs
VLAN implementation combines Layer 2 switching and Layer 3 routing
technologies to limit both collision domains and broadcast domains.

VLANs can also be used to provide security by creating the VLAN
groups according to function and by using routers to communicate
between VLANs.

A physical port association is used to implement VLAN assignment.

Communication between VLANs can occur only through the router.

This limits the size of the broadcast domains and uses the router to
determine whether one VLAN can talk to another VLAN.

NOTE: This is the only way a switch can break up a broadcast domain!
516

Setting up VLAN Implementation

517

VLAN Communication

518

VLAN Membership Modes

• VLAN membership can either be static or dynamic.
519

Static VLANs

• All users attached to same switch port must be in the same VLAN.
520

Configuring VLANs in Global
Mode
Switch#configure terminal
Switch(config)#vlan 3
Switch(config-vlan)#name Vlan3
Switch(config-vlan)#exit
Switch(config)#end

521

Configuring VLANs
in VLAN Database Mode
Switch#vlan database
Switch(vlan)#vlan 3

VLAN 3 added:
Name: VLAN0003
Switch(vlan)#exit
APPLY completed.
Exiting....

522

Deleting VLANs in Global Mode

Switch(config)#no vlan 3
Switch(config)#end

523

Deleting VLANs
in VLAN Database Mode

Switch#vlan database
Switch(vlan)#no vlan 3

VLAN 3 deleted:
Name: VLAN0003
Switch(vlan)#exit
APPLY completed.
Exiting....

524

Assigning Access Ports to a
VLAN
Switch(config)#interface gigabitethernet 1/1

• Enters interface configuration mode

Switch(config-if)#switchport mode access

• Configures the interface as an access port

Switch(config-if)#switchport access vlan 3

• Assigns the access port to a VLAN

525

Verifying the VLAN
Configuration
Switch#show vlan [id | name] [vlan_num | vlan_name]

VLAN Name Status Ports
---- -------------------------------- --------- -------------------------------
1 default active Fa0/1, Fa0/2, Fa0/5, Fa0/7
Fa0/8, Fa0/9, Fa0/11, Fa0/12
Gi0/1, Gi0/2
2 VLAN0002 active
51 VLAN0051 active
52 VLAN0052 active
…

VLAN Type SAID MTU Parent RingNo BridgeNo Stp BrdgMode Trans1 Trans2
---- ----- ---------- ----- ------ ------ -------- ---- -------- ------ ------
1 enet 100001 1500 - - - - - 1002 1003
2 enet 100002 1500 - - - - - 0 0
51 enet 100051 1500 - - - - - 0 0
52 enet 100052 1500 - - - - - 0 0
…

Remote SPAN VLANs
------------------------------------------------------------------------------
Primary Secondary Type Ports 526
------- --------- ----------------- ------------------------------------------

Verifying the VLAN Port
Configuration
Switch#show running-config interface {fastethernet |
gigabitethernet} slot/port

• Displays the running configuration of the interface

Switch#show interfaces [{fastethernet | gigabitethernet}
slot/port] switchport

• Displays the switch port configuration of the interface

Switch#show mac-address-table interface interface-id [vlan
vlan-id] [ | {begin | exclude | include} expression]

• Displays the MAC address table information for the specified
interface in the specified VLAN
527

Importance of Native VLANs

530

ISL Encapsulation

– Performed with ASIC
– Not intrusive to client
stations; client does not
see the header
– Effective between
switches, and between
routers and switches

531

ISL and Layer 2 Encapsulation

532

Configuring ISL Trunking
Switch(config)#interface fastethernet 2/1

• Enters interface configuration mode

Switch(config-if)#switchport mode trunk

• Configures the interface as a Layer 2 trunk

Switch(config-if)#switchport trunk encapsulation [isl|dot1q]

• Selects the encapsulation

533

Verifying ISL Trunking

Switch#show interfaces [fastethernet | gigabitethernet]
slot/port [ switchport | trunk ]

Switch#show interfaces fastethernet 2/1 trunk

Port Mode Encapsulation Status Native VLAN
Fa2/1 desirable isl trunking 1

Port VLANs allowed on trunk
Fa2/1 1-1005

Port VLANs allowed and active in management domain
Fa2/1 1-2,1002-1005

Port VLANs in spanning tree forwarding state and not pruned
Fa2/1 1-2,1002-1005

534

802.1Q Trunking

535

Configuring 802.1Q Trunking

Switch(config)#interface fastethernet 5/8
Switch(config-if)#shutdown
Switch(config-if)#switchport trunk encapsulation dot1q
Switch(config-if)#switchport trunk allowed vlan 1,15,11,1002-1005
Switch(config-if)#switchport mode trunk
Switch(config-if)#switchport nonegotiate
Switch(config-if)#no shutdown

536

Verifying 802.1Q Trunking

Switch#show interfaces [fastethernet | gigabitethernet]
slot/port [ switchport | trunk ]

Switch#show interfaces gigabitEthernet 0/1 switchport
Name: Gi0/1
Switchport: Enabled
Administrative Mode: trunk
Operational Mode: trunk
Administrative Trunking Encapsulation: dot1q
Operational Trunking Encapsulation: dot1q
Negotiation of Trunking: On
Access Mode VLAN: 1 (default)
Trunking Native Mode VLAN: 1 (default)
Trunking VLANs Enabled: ALL
Pruning VLANs Enabled: 2-1001

. . .
537

VTP Protocol Features
– Advertises VLAN configuration information
– Maintains VLAN configuration consistency throughout a
common administrative domain
– Sends advertisements on trunk ports only

539

VTP Modes
• Creates, modifies, and deletes
VLANs
• Sends and forwards
advertisements
• Synchronizes VLAN
configurations
• Saves configuration in NVRAM

• Cannot create,
change, or delete • Creates, modifies, and
VLANs deletes VLANs locally
• Forwards only
advertisements • Forwards
• Synchronizes advertisements
VLAN • Does not
configurations synchronize VLAN
• Does not save in configurations
NVRAM • Saves configuration in
NVRAM 540

VTP Operation
• VTP advertisements are sent as multicast frames.
• VTP servers and clients are synchronized to the latest update identified
revision number.

• VTP advertisements are sent every 5 minutes or when there is a change.

541

VTP Pruning

• Increases available bandwidth by reducing unnecessary flooded traffic
• Example: Station A sends broadcast, and broadcast is flooded only toward
any switch with ports assigned to the red VLAN.

542

VTP Configuration Guidelines
– Configure the following:
• VTP domain name
• VTP mode (server mode is the default)
• VTP pruning
• VTP password

– Be cautious when adding a new switch into an existing
domain.
– Add a new switch in a Client mode to get the last up-to-
date information from the network then convert it to
Server mode.
– Add all new configurations to switch in transparent mode
and check your configuration well then convert it to
Server mode to prevent the switch from propagating
incorrect VLAN information.
543

Configuring a VTP Server

Switch(config)#vtp server

• Configures VTP server mode

Switch(config)#vtp domain domain-name

• Specifies a domain name

Switch(config)#vtp password password

• Sets a VTP password

Switch(config)#vtp pruning

• Enables VTP pruning in the domain
544

Configuring a VTP Server
(Cont.)


Switch(config)#vtp server

Setting device to VTP SERVER mode.
Switch(config)#vtp domain Lab_Network

Setting VTP domain name to Lab_Network
Switch(config)#end

545

Verifying the VTP Configuration
Switch#show vtp status

Switch#show vtp status

VTP Version : 2
Configuration Revision : 247
Maximum VLANs supported locally : 1005
Number of existing VLANs : 33
VTP Operating Mode : Client
VTP Domain Name : Lab_Network
VTP Pruning Mode : Enabled
VTP V2 Mode : Disabled
VTP Traps Generation : Disabled
MD5 digest : 0x45 0x52 0xB6 0xFD 0x63 0xC8 0x49 0x80
Configuration last modified by 0.0.0.0 at 8-12-99 15:04:49
Switch#

546

Verifying the VTP Configuration
(Cont.)
Switch#show vtp counters

Switch#show vtp counters

VTP statistics:
Summary advertisements received : 7
Subset advertisements received : 5
Request advertisements received : 0
Summary advertisements transmitted : 997
Subset advertisements transmitted : 13
Request advertisements transmitted : 3
Number of config revision errors : 0
Number of config digest errors : 0
Number of V1 summary errors : 0

VTP pruning statistics:
Trunk Join Transmitted Join Received Summary advts received from
non-pruning-capable device
---------------- ---------------- ---------------- ---------------------------
Fa5/8 43071 42766 5
547

Contents
• Remote access overview
• WAN Connection Types
• Defining WAN Encapsulation Protocols
• Determining the WAN Type to Use
• OSI Layer-2 Point-to-Point WANs
– PPP
– HDLC
– Frame Relay
549

Remote Access Overview
• A WAN is a data communications network
covering a relatively broad geographical
area.

• A network administrator designing a
remote network must weight issues
concerning users needs such as
bandwidth and cost of the variable
available technologies.
550

WAN Connection Types

551


• Leased lines
– It is a pre-established WAN communications path
from the CPE, through the DCE switch, to the CPE
of the remote site, allowing DTE networks to
communicate at any time with no setup procedures
before transmitting data.
• Circuit switching
– Sets up line like a phone call. No data can transfer
before the end-to-end connection is established.

552

• Packet switching
– WAN switching method that allows you to share
bandwidth with other companies to save money. As
long as you are not constantly transmitting data and
are instead using bursty data transfers, packet
switching can save you a lot of money.

– However, if you have constant data transfers, then
you will need to get a leased line.
– Frame Relay and X.25 are packet switching
technologies.
553

Defining WAN Encapsulation
Protocols
• Each WAN connection uses an
encapsulation protocol to encapsulate
traffic while it crossing the WAN link.

• The choice of the encapsulation protocol
depends on the underlying WAN
technology and the communicating
equipment.
554

Defining WAN Encapsulation
Protocols
• Typical WAN encapsulation types include the
following:

– Point-to-Point Protocol (PPP)
– Serial Line Internet Protocol (SLIP)
– High-Level Data Link Control Protocol (HDLC)
– X.25 / Link Access Procedure Balanced (LAPB)
– Frame Relay
– Asynchronous Transfer Mode (ATM)

555

Determining the WAN Type to
Use
• Availability
– Each type of service may be available in certain
geographical areas.
• Bandwidth
– Determining usage over the WAN is important to
evaluate the most cost-effective WAN service.
• Cost
– Making a compromise between the traffic you need to
transfer and the type of service with the available cost
that will suit you.

556

Determining the WAN Type to
Use
• Ease of Management
– Connection management includes both the
initial start-up configuration and the outgoing
configuration of the normal operation.
• Application Traffic
– Traffic may be as small as during a terminal
session , or very large packets as during file
transfer.

557

Max. WAN Speeds for WAN
Connections

Maximum
WAN Type
Speed
Asynchronous Dial-Up 56-64 Kbps

X.25, ISDN – BRI 128 Kbps

ISDN – PRI E1 / T1

Leased Line / Frame Relay E3 / T3
558

OSI Layer-2 Point-to-Point
WANs
• WAN protocols used on Point-to-Point
serial links provide the basic function of
data delivery across that one link.

• The two most popular data link protocols
used today are Point-to-Point Protocol
(PPP) and High-Level Data Link Control
(HDLC).
559

HDLC
• HDLC performs OSI Layer-2 functions.
• It determines when it is appropriate to use
the physical medium.
• Ensures that the correct recipient receives
and processes the data that is sent.
• Determines whether the sent data was
received correctly or not (error detection).

560

HDLC
• HDLC Frame Format

• The original HDLC didn’t include any
Protocol Type field, every company
(including Cisco) added its own field, so it
became a proprietary protocol that can be
used between only Cisco routers.
561

Point-to-Point Protocol (PPP)
• PPP is a standard encapsulation protocol for the
transport of different Network Layer protocols
(including, but not limited to, IP).

• It has the following main functional components
– Link Control Protocol (LCP) that
establishes, authenticates, and tests the data link
connection.
– Network Control Protocols (NCPs) that establishes
and configure different network layer protocols.
562

Point-to-Point Protocol (PPP)
• PPP discards frames that do not pass the
error check.
• PPP is a standard protocol, and so it can
be used with all types of routers (not Cisco
Proprietary).

563

PPP LCP Features
• Authentication
• Compression
• Multilink PPP
• Error Detection
• Looped Link Detection

564

Compression
• Compression enables higher data throughput
across the link.
• Different compression schemes are available:
– Predictor : checks if the data was already
compressed.
– Stacker : it looks at the data stream and only sends
each type of data once with information about where
the type occurs and then the receiving side uses this
information to reassemble the data stream.
– MPPC (Microsoft Point-to-Point Compression) :
allows Cisco routers to compress data with Microsoft
clients.
567

PPP Multilink
• PPP Multilink provides load balancing over
dialer interfaces-including ISDN,
synchronous, and asynchronous
interfaces.

• This can improve throughput and reduce
latency between systems by splitting
packets and sending fragments over
parallel circuits.
568

Error Detection
• PPP can take down a link based on the
value of what is called LQM (Link Quality
Monitor) as it gets the ratio of corrupted
packets to the total number of sent
packets, and according to a predetermined
value, the link can be brought down if it is
thought that its performance is beyond
limits accepted.

569

Looped Link Detection
• PPP can detect looped links (that are
sometimes done by Teleco companies)
using what is called Magic Number.

• Every router will have a magic
number, and if packets were received
having the same router’s magic
number, then the link is looped.
570

PPP Configuration Commands
• To enable PPP
– Router(config-if)#encapsulation ppp

• To configure PAP authentication
– Router(Config-if)#ppp authentication pap
– Router(Config-if)#ppp pap username .. password ..

• To configure Compression
– Router(Config-if)#compress [predictor|stack|mppc]

571

Frame Relay

572

Frame Relay
• Frame Relay networks use permanent virtual circuits
(PVCs) or switched virtual circuits (SVCs) but most
nowadays Frame Relay networks use permanent virtual
circuits (PVCs).
• The logical path between each pair of routers is called a
Virtual Circuit (VC).
• VCs share the access link and the frame relay network.
• Each VC is committed to a CIR (Committed Information
Rate) which is a guarantee by the provider that a
particular VC gets at least this much of BW.

575

PVC
PC PVC

Port PVC
CPE SVC
UNI
SVC
Controller
Router ISDN dial-up connection
or Switch
direct connection
(V.35, E1, RS232)
PBX

Video

Desktop & LAN Network access Frame Relay
Network
Formats
packets
in frames

576

LMI and Encapsulation Types
• The LMI is a definition of the messages used
between the DTE and the DCE.

• The encapsulation defines the headers used by
a DTE to communicate some information to the
DTE on the other end of a VC.

• The switch and its connected router care about
using the same LMI; the switch does not care
about the encapsulation. The endpoint routers
(DTEs) do care about the encapsulation.
577

LMI
• The most important LMI message is the LMI
status inquiry message. Status messages
perform two key functions:

– Perform a keepalive function between the DTE and
DCE. If the access link has a problem, the absence of
keepalive messages implies that the link is down.

– Signal whether a PVC is active or inactive. Even
though each PVC is predefined, its status can
change.

578

LAPF
• A Frame Relay-connected router encapsulates
each Layer 3 packet inside a Frame Relay header
and trailer before it is sent out an access link.

• The header and trailer are defined by the Link
Access Procedure Frame Bearer Services (LAPF)
specification.

• The LAPF framing provides error detection with
an FCS in the trailer, as well as the DLCI, DE,
FECN, and BECN fields in the header.
580

LAPF
• DTEs use and react to the fields specified by
these two types of encapsulation, but Frame
Relay switches ignore these fields. Because the
frames flow from DTE to DTE, both DTEs must
agree to the encapsulation used.

• However, each VC can use a different
encapsulation. In the configuration, the
encapsulation created by Cisco is called
cisco, and the other one is called ietf.

581

DLCI Addressing Details
• The logical path between a pair of DTEs is called a
virtual circuit (VC).
• The data-link connection identifier (DLCI) identifies
each individual PVC.
• When multiple VCs use the same access link, the
Frame Relay switches know how to forward the
frames to the correct remote sites.

The DLCI is the Frame Relay address describing
a Virtual Circuit
582

DLCI=17 B
DLCI=32
DLCI=32

DLCI=16
FR-network
R
DLCI=17
DLCI=16 DLCI=16
DLCI=21 R

Virtual circuit

R Router

B Bridge

Frame Relay switch

583

DLCI Addressing Details
• The difference between layer-2 addressing
and DLCI addressing is mainly because
the fact that the header has a single DLCI
field, not both Source and Destination
DLCI fields.

584

Global DLCI Addressing
• Frame Relay DLCIs are locally significant; this
means that the addresses need to be unique
only on the local access link.
• Global addressing is simply a way of choosing
DLCI numbers when planning a Frame Relay
network so that working with DLCIs is much
easier.
• Because local addressing is a fact, global
addressing does not change these rules. Global
addressing just makes DLCI assignment more
obvious.
585

Global DLCI Addressing
• The final key to global addressing is that the
Frame Relay switches actually change the DLCI
value before delivering the frame.
• The sender treats the DLCI field as a
destination address, using the destination’s
global DLCI in the header.
• The receiver thinks of the DLCI field as the
source address, because it contains the global
DLCI of the frame’s sender.

587

Layer 3 Addressing
• Cisco’s Frame Relay implementation
defines three different options for
assigning subnets and IP addresses on
Frame Relay interfaces:
– One subnet containing all Frame Relay DTEs
– One subnet per VC
– A hybrid of the first two options

588

One Subnet Containing All Frame
Relay DTEs
• The single-subnet option is typically used
when a full mesh of VCs exists.

• In a full mesh, each router has a VC to
every other router, meaning that each
router can send frames directly to every
other router

589

One Subnet Per VC
• The single-subnet-per-VC alternative, works better with a
partially meshed Frame Relay network.

592

Hybrid Terminology
• Point-to-point subinterfaces are used when a
single VC is considered to be all that is in the
group—for instance, between Routers A and D
and between Routers A and E.

• Multipoint subinterfaces are used when more
than two routers are considered to be in the
same group— for instance, with Routers
A, B, and C.

594

Frame Relay Address Mapping
• Mapping creates a correlation between a Layer-
3 address (IP Address) and its corresponding
Layer-2 address (DLCI in Frame Relay).

• It is used so that after the router receives the
packet with the intended IP address could be
able to handle it to the right Frame Relay switch
(with the appropriate DLCI)

597

Mapping Methods
• Mapping can be done either two ways:
• Dynamic Mapping
– Using the Inverse ARP that is enabled by
default on Cisco routers.

• Static Mapping
– Using the frame-relay map command but you
should first disable the inverse arp using the
command no frame-relay inverse-arp

598

Integrated Services Digital
Network (ISDN)

602

LAPD & PPP on D and B
Channels
• LAPD is used as a data-link protocol across an
ISDN D channel.

• Essentially, a router with an ISDN interface
needs to send and receive signaling messages
to and from the local ISDN switch to which it is
connected.

• LAPD provides the data-link protocol that allows
delivery of messages across that D channel to
the local switch.
606

Channels
• The call setup and teardown messages
themselves are defined by the Q.931
protocol. So, the local switch can receive a
Q.931 call setup request from a router
over the LAPD-controlled D channel, and it
should react to that Q.931 message by
setting up a circuit over the public network.

607

Channels
• An ISDN switch often requires some form of
authentication with the device connecting to it.

• Switches use a free-form decimal value, call the
service profile identifier (SPID), to perform
authentication.

• In short, before any Q.931 call setup messages
are accepted, the switch asks for the configured
SPID values. If the values match what is
configured in the switch, call setup flows are
accepted. 608

PRI Encoding and Framing
• ISDN PRI in North America is based on a digital
T1 circuit. T1 circuits use two different encoding
schemes—Alternate Mark Inversion (AMI) and
Binary 8 with Zero Substitution (B8ZS).

• The two options for framing on T1s are to use
either Extended Super Frame (ESF) or the older
option—Super Frame (SF). In most cases today,
new T1s use ESF.

609

DDR (Dial On Demand Routing)
• You can configure DDR in several
ways, including Legacy DDR and DDR dialer
profiles.

• The main difference between the two is that
Legacy DDR associates dial details with a
physical interface, whereas DDR dialer profiles
disassociate the dial configuration from a
physical interface, allowing a great deal of
flexibility.
610

Legacy DDR Operation
1. Route packets out the interface to be dialed.
2. Determine the subset of the packets that
trigger the dialing process.
3. Dial (signal).
4. Determine when the connection is
terminated.

611

DDR Step 1: Routing Packets Out the
Interface to Be Dialed
• DDR does not dial until some traffic is directed
(routed) out the dial interface.
• The router needs to route packets so that they are
queued to go out the dial interface. Cisco’s design for
DDR defines that the router receives some user-
generated traffic and, through normal routing
processes, decides to route the traffic out the interface
to be dialed.
• The router (SanFrancisco) can receive a packet that
must be routed out BRI0; routing the packet out BRI0
triggers the Cisco IOS software, causing the dial to
occur.
613

DDR Step 2:
Determining the Interesting Traffic
• Packets that are worthy of causing the device to
dial are called interesting packets.

• Two different methods can be used to define
interesting packets.
– In the first method, interesting is defined as all
packets of one or more Layer 3 protocols.
– The second method allows you to define packets as
interesting if they are permitted by an access list.

614

DDR Step 3:
Dialing (Signaling)
• Defining the phone number to be dialed.

• The command is dialer string , where
string is the phone number (used when
dialing only one site).

• The dialer map command maps the
different dialer numbers to the equivalent
IP addresses of the routers to be dialed.
615

Configuring SPIDs
• You might need to configure the Service Profile
Identifier (SPID) for one or both B
channels, depending on the switch’s
expectations.

• When the telco switch has configured SPIDs, it
might not allow the BRI line to work unless the
router announces the correct SPID values to the
switch. SPIDs, when used, provide a basic
authentication feature.
616

ISDN PRI Configuration
1. Configure the type of ISDN switch to which this
router is connected.
2. Configure the T1 or E1 encoding and framing
options (controller configuration mode).
3. Configure the T1 or E1 channel range for the
DS0 channels used on this PRI (controller
configuration mode).
4. Configure any interface settings (for example,
PPP encapsulation and IP address) on the
interface representing the D channel.

617

Configuring a T1 or E1 Controller
• Your service provider will tell you what
encoding and framing to configure on the
router. Also, in almost every case, you will
use all 24 DS0 channels in the PRI—23 B
channels and the D channel.

620

DDR With Dialer Profiles
• Dialer profiles pool the physical interfaces
so that the router uses any available B
channel on any of the BRIs or PRIs in the
pool.

• Dialer profiles configuration moves most of
the DDR interface configuration to a virtual
interface called a dialer interface.
621

With all my best wishes for you
to succeed and distinguish in the
CCNA International Exam,
Keep In touch


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