VLAN

Virtual Local Area
Network (VLAN)
Prepared by: Roshan Kandel
Masters in Information & Communication Engineering
1

Introduction
• In a layer 2 switched network, each network segment has its
own collision domain and all segments are in same broadcast domain.
Every broadcast is seen by every device on the network.
• A layer 3 device (typically a Router) is used to segment (divide)
a broadcast domain to multiple broadcast domains.
• By default, all ports on a switch are in the same broadcast domain.
• A Virtual Local Area Network, Virtual LAN, or VLAN, can be used to
segment (divide) a single broadcast domain to multiple broadcast
domains in a layer 2 switched network.
• VLANs are not restricted to any physical boundary in the switched
network if the devices are interconnected using switches.
2

• A VLAN can span across multiple switches, or it can be limited within
a switch.
• A VLAN must be configured as a separate Layer 3 IP subnet and you
need a Layer 3 device (typically a Router) to enable communication
between different VLANs.
• Therefor a VLAN = Broadcast Domain = A Separate IP subnet
3

Advantages of Virtual Local Area Network
(VLAN)
• The main advantages of VLAN are listed below.
• Broadcasts are required for the normal function of a network. Many
protocols and applications depend on broadcast communication to
function properly.
• A layer 2 switched network is in a single broadcast domain and the
broadcasts can reach the network segments which are so far where a
particular broadcast has no scope and consume available network
bandwidth.
• If we segment a large LAN to smaller VLANs we can reduce
broadcast traffic as each broadcast will be sent on to the relevant
VLAN only.
4

• Security: VLANs provide enhanced network security.
• In a VLAN network environment, with multiple broadcast domains,
network administrators have control over each port and user.
• A malicious user can no longer just plug their workstation into any
switch port and sniff the network traffic using a packet sniffer.
• The network administrator controls each port and whatever resources
it is allowed to use.
• Cost: Segmenting a large VLAN to smaller VLANs is cheaper than
creating a routed network with routers because normally routers
costlier than switches.
5

Static VLAN and Dynamic VLAN
• Two types of VLAN membership methods exists and they are Static
and Dynamic.
• Static VLANs: In a static VLAN, the network administrator creates a
VLAN and then assigns switch ports to the VLAN. Static VLANs are
also called port-based VLANs.
• The association with the VLAN does not change until the
administrator changes the port assignment.
• End-user devices become the members of VLAN based on the
physical switch port to which they are connected.
6

• The ports on a single switch can be assigned multiple VLANs.
• Even though two devices are connected to different ports on a same
switch, traffic will not pass between them if the connected ports are on
different VLANs.
• We need a layer 3 device (typically a Router) to enable communication
between two VLANs.
• Dynamic VLANs: In a dynamic VLAN, the switch automatically
assigns the port to a VLAN using information from the user device
like MAC address, IP address etc.
7

• When a device is connected to a switch port, the switch queries a
database to establish VLAN membership.
• A network administrator must configure VLAN database of a VLAN
Membership Policy Server (VMPS).
• Dynamic VLANs support instant movability of end devices. When we
move a device from a port on one switch to a port on another switch,
the dynamic VLANs will automatically configure the membership of
the VLAN.
8

How to create and name static VLAN and
IOS commands to create VLAN
• To create and name a static VLAN, open console connection to Switch 1,
and move to global configuration mode. Enter the following IOS commands
to create VLAN 10 and name VLAN 10 as 'engineering'.
• SW1>enable
• SW1#configure terminal
• Enter configuration commands, one per line. End with CNTL/Z.
• SW1(config)#vlan 10
• SW1(config-vlan)#name engineering
• SW1(config-vlan)#exit
• SW1(config)#exit
• SW1#
9

• To delete the VLAN, just use the no form of the above command.
• SW1(config)#no vlan 10
10

How to view VLAN information using 'show
vlan' IOS command
• To view VLAN configuration information, open console connection to
switch and run the "show vlan" IOS command from privileged mode
as shown below.
• SW1>enable
• SW1#show vlan
11

Types of VLAN connection links - Trunk
Links and Access Links
• There are two types of VLAN connection links and they are Access
link and Trunk link.
• Access link: An access link is a link that is part of only one VLAN,
and normally access links are for end devices.
• Any device attached to an access link is unaware of a VLAN
membership.
• An access-link connection can understand only standard Ethernet
frames.
• Switches remove any VLAN information from the frame before it is
sent to an access-link device.
12

• Trunk link: A Trunk link can carry multiple VLAN traffic and
normally a trunk link is used to connect switches to other switches or
to routers.
• To identify the VLAN that a frame belongs to, Cisco switches support
different identification techniques (VLAN Frame tagging).
• Our focus is on IEEE 802.1Q.
• A trunk link is not assigned to a specific VLAN. Many VLAN traffic
can be transported between switches using a single physical trunk link.
13

Trunk Links and Access Links
14
• The access links are part of
only one VLAN and carry
traffic to only the end
devices connected to that
particular VLAN.
• But a trunk link is used to
connect switches to other
switches or to routers and
can carry traffic from
multiple VLANs.

VLAN Frame tagging
• VLAN Frame tagging is a technology which is used to identify the
VLAN that the packet belongs to.
• The VLAN Frame tag is placed on the Ethernet frame when the
Ethernet frame reaches a switch from an access port, which is a
member of a VLAN.
• If the switch has a trunk port, the Ethernet frame can be forwarded out
the trunk link port.
• This enables each switch to see what VLAN the Ethernet frame
belongs to and can forward the Ethernet frame to corresponding
VLAN access ports or to another VLAN trunk port.
15

• Before forwarding the frame to a VLAN access port, the switch
removes the VLAN identifier and the VLAN membership information
is hence transparent to the end devices.
• There are different VLAN frame trunking technologies available.
• Inter-Switch Link (ISL): Cisco proprietary VLAN frame tagging. No
much support from other vendors. Supported only in old Cisco Switch
models.
• IEEE 802.1Q: IEEE industry standard VLAN frame tagging
• (More information on Dot1Q:
https://en.wikipedia.org/wiki/IEEE_802.1Q)
16

What is Native VLAN
• Normally a Switch port configured as a trunk port send and receive
IEEE 801.q VLAN tagged Ethernet frames.
• If a switch receives untagged Ethernet frames on its Trunk port, they
are forwarded to the VLAN that is configured on the Switch as native
VLAN.
• Both sides of the trunk link must be configured to be in same native
VLAN.
17

How to configure VLAN trunk link and
native VLAN
• By default, all switch ports in Layer 2 are configured to operate as
access links.
• To configure the trunk link, open console connection to Switch 1 and
enter the commands as shown below.
• SW1>enable
• SW1(config-if)#switchport trunk encapsulation dot1q
• SW1(config-if)#switchport mode trunk
• SW1(config-if)#switchport trunk native vlan 10
18

• To configure trunk link and native VLAN on Switch 2, open console
connection to Switch 2 and enter the commands as shown below.
• SW2>enable
• SW2(config-if)#switchport mode trunk
• SW2(config-if)#switchport trunk native vlan 10
19

How to configure and assign a Cisco switch
access port to a VLAN
• Access ports in switches are normally used to connect end devices like
workstations and printers.
• Access ports belong to only a single VLAN and do not provide any
identifying marks on the Ethernet frames.
• When configuring an access port, you also want to define which
VLAN the port belongs to.
• To configure and assign a switch access port to a VLAN, open a
console connection to the switch and run the following IOS commands
from interface configuration mode.
20

• SW1>enable
• SW1(config)#interface fa0/1
• SW1(config-if)#switchport mode access
• SW1(config-if)#switchport access vlan 10
• SW1(config-if)#exit
• SW1(config)#interface fa0/10
• SW1(config-if)#switchport mode access
• SW1(config-if)#switchport access vlan 20
21

What is VLAN Trunking Protocol (VTP)
• For a small network VLAN configuration and VLAN trunking
configuration is easy to manage.
• But configuring VLAN and VLAN trunking for large networks with
many interconnected switches can be a very difficult task.
• VLAN Trunk Protocol (VTP) is a protocol created by Cisco to create
and manage VLANs for a large network with many interconnected
switches and to maintain consistency throughout the network.
• In other words, switches use VLAN trunking protocol (VTP) to
communicate among themselves about VLAN configuration.
22

• The VLAN Trunking Protocol (VTP) is a very useful protocol to
create, manage and maintain a large network with many
interconnected switches.
• The VLAN Trunking Protocol (VTP) can manage the addition,
deletion, and renaming of VLANs from a central point without manual
intervention and VLAN Trunk Protocol (VTP) thus reduces network
administration in a switched network.
23

Scenario: The University School
24
• Suppose you work for a school at a university, and the school is in the process of
moving to a different building. In the new building, administrative operations and
academic labs will be located on the same floor. In the interest of saving money
and time, it has been decided that all of the school's devices will be connected via
a single network switch. Since security of the administrative devices is important,
the administrative network must be physically or virtually separated from the
academic network.
• The administrative network devices will be assigned into VLAN 100, and all
academic network devices will be assigned into VLAN 200. On the switch, all
administrative devices will be connected to switchports Fast Ethernet 0/1–0/12,
and all academic devices will be connected to switchports Fast Ethernet 0/13–
0/24.
• To set up this design, each interface must be configured into its respective VLAN.

Inter-VLAN Routing
• A VLAN is a broadcast domain, which means computers on separate
VLANs are unable to communicate without the intervention of a
routing device.
• Whenever hosts in one VLAN need to communicate with hosts in
another VLAN, the traffic must be routed through a routing device.
• This process is known as inter-VLAN routing.
• To successfully exchange information between VLANs, you need a
router or a Layer 3 switch.
25

• There are three possible ways to implement inter-VLAN routing:
• 1. Traditional Inter-VLAN Routing
• 2. Router-on-a-Stick Inter-VLAN Routing
• 3. Multilayer Switch Inter-VLAN Routing
26

1. Traditional Inter-VLAN Routing
• This method of inter-VLAN routing relies on a router with multiple
physical interfaces.
• Each interface is usually connected to the switch, one for each VLAN.
• The switch ports connected to the router are placed in access mode and
each router interface can then accept traffic from the VLAN associated
with the switch interface that it is connected to, and traffic can be
routed to the other VLANs connected to the other interfaces.
• This means that each of the routers’ interface IP addresses would then
become the default gateway address for each host in each VLAN.
27

• Let’s take a look at the diagram shown below.
• If Host A on VLAN 10, wants to send a message to Host B on VLAN
20, it would take the following steps:
28

Steps
• Host A checks whether the destination IP address is in its VLAN; if it is not, the
traffic will be forwarded to its default gateway on interface Fa0/0 on the router.
• Host A then sends an ARP request to the switch to determine the MAC address of
the Fa0/0 interface on the router. Once the router replies, Host A sends the frame
to the router as a unicast message, where it is then directly forwarded out the trunk
interface to the router.
• When the router receives the frame, it determines the destination IP address and
interface from the routing table.
• The router then sends an ARP request out the interface connected to the
destination VLAN (VLAN 20), which corresponds to interface Fa0/1 on the
router.
• When the switch receives the message, it floods it to its ports, which then triggers
Host B to reply with its MAC address.
• The router then uses the information gathered to forward the message finally to
Host B on VLAN 20 as a unicast frame through the switch.
29

• Step 1: Create VLANs (VLANs 10 and 20) on the switch
• Step 2: Assign the VLANs to switch port
• Step 3: Configure the IP addresses on the router
• Now at this juncture, if you try to ping between Host A and Host B, it will
be successful because the two VLANs are now interconnected through the
router.
• Traditional inter-VLAN routing happens to be the earliest form of inter-
VLAN routing. However, this method of inter-VLAN routing is not
efficient.
• It is archaic (Old-fashioned), and no longer employed in today’s switched
networks.
30

• This is because routers have a limited number of physical interfaces
that can be used to connect to different VLANs.
• Therefore, as the number of VLANs increases on a network, the
approach of having one router physical interface per VLAN becomes
unsustainable due to the inherent hardware limitations of a router.
• In order to overcome some of the issues associated with traditional
inter-VLAN routing, a new method known as router-on-a-stick was
invented.
31

2. Router-on-a-Stick Inter-VLAN Routing
• A router-on-a-stick is a method of inter-VLAN routing in which the
router is connected to the switch using a single physical interface,
hence the name router-on-a-stick.
• Most modern inter-VLAN routing implementations are designed using
this method.
• Unlike the traditional inter-VLAN routing method, router-on-stick
does not require multiple physical interfaces on both the router and the
switch.
• Instead, the router’s operating system makes it possible to configure
the router interface to operate as a trunk link, which is then connected
to a switch port that is configured in trunk mode.
32

• This implies that only one physical interface is required on the router
and the switch to route packets between multiple VLANs.
• IEEE 802.1Q (Dot1q) protocol—which defines a system of VLAN
tagging for Ethernet frames, is used to provide multi-vendor VLAN
support.
• The single physical interface on the router is linked to logical (virtual)
subinterfaces, which can be configured with multiple IP addresses that
correspond to the VLANs on the switch.
33

• Each subinterface is configured for different subnets corresponding to
their VLAN assignment to facilitate logical routing.
• The router performs inter-VLAN routing by accepting traffic from all
the VLANs.
• It then determines the destination network based on the source and
destination IP in the packets.
• After a routing decision is made based on the destination VLAN, it
then forwards the data frames to the switch with the correct VLAN
information through the same physical interface used to receive the
traffic.
34

• Let’s take a look at the diagram shown below.
• If Host A on VLAN 10, wants to send a message to Host B on VLAN
20, the steps it would take are as follows:
35

Steps
• Host A sends its unicast traffic to the switch.
• The switch then tags the unicast traffic as originating on VLAN 10 and
forwards it out its trunk link to the router.
• The router accepts the tagged unicast traffic on VLAN 10 and routes it
to VLAN 20 using its configured subinterfaces.
• The unicast traffic is tagged with VLAN 20 as it is sent out the router
interface to the switch.
• The switch removes the VLAN tag of the unicast frame and forwards
the frame directly to Host B on port Fa0/3.
36

• Step 1: Create VLANs (VLANs 10 and 20) on the switch
• Step 2: Assign the VLANs to switch ports
• Step 3: Configure the IP addresses on the router
37
Enter global configuration mode Router# conf t
Enter sub-interface config. mode for fa0/1.10 Router(config)# interface fa0/1.10
Set encapsulation type to 802.1Q and assign
VLAN 10 to the virtual interface
Router(config-subif)# encapsulation dot1Q 10
Configure IP address and subnet mask
Router(config-subif)#ip address 192.168.10.1
255.255.255.0
Exit the sub-interface Router(config-subif)#exi

38
Enter sub-interface config. mode for fa0/1.20 Router(config)# interface fa0/1.20
Set the encapsulation type to 802.1Q and
assign VLAN 20 to the virtual interface.
Router(config-subif)# encapsulation dot1Q
20
Configure IP address and subnet mask
Router(config-subif)# ip address
192.168.20.1 255.255.255.0
Exit the sub-interface Router(config-subif)#exit

• A ping between Host A and Host B will be successful because the two
VLANs are now interconnected through the router.
• The router-on-a-stick method of inter-VLAN routing also has some
limitations, such as scalability and latency issues.
• To overcome these issues, Cisco developed a better alternative: The
Multilayer Switch Inter-VLAN Routing.
39

3. Multilayer Switch Inter-VLAN Routing
• Multilayer Switch Inter-VLAN Routing is a method of inter-VLAN
routing in which a different kind of switch known as a multilayer
switch is used to perform routing functions.
• A multilayer switch is a hybrid device that combines the functions of a
switch with a router, which enables it to operate on both Layer 2 (L2)
and Layer 3 (L3) of the OSI model, hence the name multilayer.
• Unlike the router-on-a-stick inter-VLAN routing method, a multilayer
switch inter-VLAN routing does not require a dedicated router—
everything happens inside the switch.
40

• Multilayer switches perform all VLAN routing functions on the
network, thereby replacing the need for dedicated routers or trunk
links.
• To enable a multilayer switch to perform routing functions, logical
(virtual) interfaces known as Switch Virtual Interface (SVI) are used,
one for each VLAN.
• SVI, also known as the VLAN interface, is a virtual routed interface
that connects a VLAN on the device to the Layer 3 routing engine
within the same device and can be configured with multiple IP
addresses that correspond to the VLANs on the switch.
41

• Each SVI is configured for different subnets corresponding to their
assigned VLAN to facilitate logical routing.
• When the multilayer switch receives a packet in a VLAN intended at
the Layer 2 switch, the multilayer switch performs routing. Let’s take
a look at the diagram shown above:
42

Steps
• If Host A in VLAN 10, wants to send a message to Host B in VLAN 20, the
steps it would take are as follows:
• Host A sends its unicast traffic to the directly connected L2 switch.
• L2 switch tags the unicast traffic as originating on VLAN 10 and forwards it
to the L3 switch via the trunk link.
• The L3 switch removes the VLAN tag and forwards the unicast traffic
internally to the VLAN 10 virtual interface.
• The L3 switch internally routes the unicast traffic to its VLAN 20 virtual
interface and then retags the traffic, which it then forwards back to the L2
switch via the trunk link.
• L2 switch removes the VLAN tag of the unicast frame and forwards the
frame directly to Host B on port fa0/3.
43

• To configure multilayer switch inter-VLAN routing on a Cisco device,
in accordance with the diagram, follow the steps below:
• Step 1: Create VLANs (VLANs 10 and 20) on the L2 switch
• Step 2: Assign the VLANs to the L2 switch ports
• Step 3: Enable L3 routing and create VLANs (VLANs 10 and 20)
on the L3 switch
44

Step 3: Enable L3 routing and create VLANs
(VLANs 10 and 20) on the L3 switch
Description Command
Enter global configuration mode L2-Switch#conf t
Enable L3 routing L3-Switch(config) # ip routing
Create VLAN 10 L3-Switch(config)#vlan 10
Give a name to VLAN 10 L3-Switch(config-vlan)# name Admin-dept
Create VLAN 20 L3-Switch(config-vlan)# vlan 20
Give a name to VLAN 20 L3-Switch(config-vlan)# name Finance-dept
Exit the VLAN config mode L3-Switch(config-vlan)# exit
Enter interface configuration for fa0/1 L3-Switch(config)# interface fa0/1
Set the encapsulation type to 802.1Q on the interface L3-Switch(config-if)# switchport trunk encapsulation dot1q
Set the port to trunk mode L3-Switch(config-if)#switchport mode trunk
45

Step 4: Configure Switch VLAN Interfaces
(SVI)
Description Command
Enter global configuration mode L3-Switch# conf t
Create a virtual interface for VLAN 10 and enter interface configuration
mode.
L3-Switch(config)# interface vlan10
Configure a static route to reach VLAN 10 L3-Switch(config-if)# ip address 192.168.10.1 255.255.255.0
Activate interface L3-Switch(config-if)# no shut
Exit the interface L3-Switch(config-if)# exit
Create a virtual interface for VLAN 20 and enter interface configuration
mode
L3-Switch(config)# interface vlan20
Configure a static route to reach VLAN 20 L3-Switch(config-if)# ip address 192.168.20.1 255.255.255.0
Activate interface L3-Switch(config-if)# no shut
46

• Again, a ping between Host A and Host B will be successful because
the two VLANs are now interconnected through the multilayer switch.
• Multilayer switch inter-VLAN routing is faster and more scalable than
any other inter-VLAN routing implementation.
• This is because routers are limited by the number of available physical
interfaces or ports, as well as the amounts of traffic that can be
accommodated on the trunk link at one time.
• However, a multilayer switch does not totally replace the functionality
of a router, as routers support a wide range of other supplementary
features and capabilities.
47

Comparison of the various inter-VLAN
routing method
Metric Traditional Inter-VLAN Routing Router-on-a-Stick Multilayer Switch
Supported routing protocol on switch Static routing Static routing Static and dynamic routing
Port Mode Access mode Trunk mode Trunk mode
Bandwidth No bandwidth contention Bandwidth contention No bandwidth contention
Latency High Medium Low
Scalability Poor Poor Excellent
Number of physical interfaces One physical interface per VLAN One physical interface for many VLANs One physical interface for many VLANs
TCO High Medium Low
Configuration Complexity Low Medium High
Acceptability Archaic – No longer implemented Modern–widely implemented Modern–widely implemented
Ideal for Legacy networks SME networks Large enterprise networks
48

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