ENARSI_Chapter_2 in PDF format easy to understand

Chapter 2: EIGRP
Instructor Materials
CCNP Enterprise: Advanced Routing

2
© 2016 Cisco and/or its affiliates. All rights reserved. Cisco Confidential
Chapter 2 Content
This chapter covers the following content:
• EIGRP Fundamentals - This section explains how EIGRP establishes a
neighborship with other routers and how routes are exchanged with other routers.
• EIGRP Configuration Modes - This section defines the two methods of
configuring EIGRP with a baseline configuration.
• Path Metric Calculation - This section explains how EIGRP calculates the path
metric to identify the best and alternate loop-free paths.

3
EIGRP Fundamentals
• Enhanced Interior Gateway Routing Protocol (EIGRP) is an enhanced distance vector
routing protocol commonly found in enterprise networks.
• EIGRP is a derivative of Interior Gateway Routing Protocol (IGRP) but includes
support for variable-length subnet masking (VLSM) and metrics capable of supporting
higher-speed interfaces.
• EIGRP overcomes the deficiencies of other distance vector routing protocols, such as
Routing Information Protocol (RIP), with features such as unequal-cost load balancing,
support for networks 255 hops away, and rapid convergence features.
• EIGRP uses a diffusing update algorithm (DUAL) to identify network paths and
provides for fast convergence using precalculated loop-free backup paths.

4
EIGRP Fundamentals
Autonomous Systems
A router can run multiple EIGRP processes. Each
process operates under the context of an
autonomous system, which represents a
common routing domain. Routers within the
same domain use the same metric calculation
formula and exchange routes only with members
of the same autonomous system.
EIGRP uses protocol-dependent modules
(PDMs) to support multiple network protocols,
such as IPv4, and IPv6. EIGRP is written so that
the PDM is responsible for the functions that
handle the route selection criteria for each
communication protocol. Current versions of
EIGRP only support IPv4 and IPv6.

5
EIGRP Fundamentals
EIGRP Terminology
Figure 2-2 is used as a reference
topology for R1 calculating the best
path and alternative loop-free paths
to the 10.4.4.0/24 network. The
values in parentheses represent the
link’s calculated metric for a
segment based on bandwidth and
delay.

6
EIGRP Fundamentals
EIGRP Terminology (Cont.)
Term Definition
Successor route The route with the lowest path metric to reach a destination. The successor route for R1 to reach
10.4.4.0/24 on R4 is R1→R3→R4.
Successor The first next-hop router for the successor route. The successor for 10.4.4.0/24 is R3.
Feasible distance (FD) The metric value for the lowest-metric path to reach a destination. The feasible distance is
calculated locally using the formula shown in the “Path Metric Calculation” section, later in this
chapter. The FD calculated by R1 for the 10.4.4.0/24 network is 3328 (that is, 256 + 256 + 2816).
Reported distance (RD) Distance reported by a router to reach a prefix. The reported distance value is the feasible distance
for the advertising router. R3 advertises the 10.4.4.0/24 prefix with an RD of 3072. R4 advertises
the 10.4.4.0/24 to R1 and R2 with an RD of 2816.
Feasibility condition For a route to be considered a backup route, the RD received for that route must be less than the
FD calculated locally. This logic guarantees a loop-free path.
Feasible successor A route with that satisfies the feasibility condition is maintained as a backup route. The feasibility
condition ensures that the backup route is loop free. The route R1→R4 is the feasible successor
because the RD of 2816 is lower than the FD of 3328 for the R1→R3→R4 path.

7
EIGRP Fundamentals
EIGRP Topology Table
EIGRP contains a topology table, which makes it different from
a true distance vector routing protocol. EIGRP’s topology table
is a vital component of DUAL and contains information to
identify loop-free backup routes. The topology table contains all
the network prefixes advertised within an EIGRP autonomous
system.
Each entry in the table contains the following:
• Network prefix
• EIGRP neighbors that have advertised that prefix
• Metrics from each neighbor (reported distance and hop
count)
• Values used for calculating the metric (load, reliability, total
delay, and minimum bandwidth)
The command show ip eigrp topology shows only the
successor and feasible successor routes, as shown in Figure
2-3, the optional all-links keyword shows all paths received.
Figure 2-3 shows the topology table for R1 from Figure 2-2.

8
EIGRP Fundamentals
EIGRP Neighbors
EIGRP does not rely on periodic advertisement of all the network prefixes in an
autonomous system, which is done with routing protocols such as Routing Information
Protocol (RIP), Open Shortest Path First (OSPF), and Intermediate System-to-
Intermediate System (IS-IS). EIGRP neighbors exchange the entire routing table when
forming an adjacency, and they advertise incremental updates only as topology changes
occur within a network.
The neighbor adjacency table is vital for tracking neighbor status and the updates sent to
each neighbor.

9
EIGRP Fundamentals
Inter-Router Communication
• EIGRP uses five different packet types to communicate with other routers, as shown in Table 2-3.
• EIGRP uses its own IP protocol number (88) and uses multicast packets where possible; it uses unicast
packets when necessary.
• Communication between routers is done with multicast using the group address 224.0.0.10 or the MAC
address 01:00:5e:00:00:0a when possible.
• EIGRP uses multicast packets to reduce bandwidth consumed on a link (one packet to reach multiple
devices).
• EIGRP uses Reliable Transport Protocol (RTP) to ensure that packets are delivered in order and to ensure
that routers receive specific packets. A sequence number is included in each EIGRP packet. The
sequence value zero does not require a response from the receiving EIGRP router; all other values require
an ACK packet that includes the original sequence number.
Table 2-3 EIGRP Packet Types
Packet Type Packet Name Function
1 Hello Used for discovery of EIGRP neighbors and for detecting when a neighbor is no longer
available
2 Request Used to get specific information from one or more neighbors
3 Update Used to transmit routing and reachability information with other EIGRP neighbors
4 Query Sent out to search for another path during convergence
5 Reply Sent in response to a query packet

10
EIGRP Fundamentals
Forming EIGRP Neighbors
Unlike other distance vector routing protocols,
EIGRP requires a neighbor relationship to
form before routes are processed and added
to the Routing Information Base (RIB). Upon
hearing an EIGRP hello packet, a router
attempts to become the neighbor of the other
router.
The following parameters must match for the
two routers to become neighbors:
• Metric formula K values
• Primary subnet matches
• Autonomous system number (ASN)
matches
• Authentication parameters
Figure 2-4 shows the process EIGRP uses for
forming neighbor adjacencies.

11
EIGRP Configuration
Modes
• The two methods of EIGRP configuration are classic mode and named mode.

12
EIGRP Configuration Mode
Classic Configuration Mode
With classic EIGRP configuration mode, most of the configuration takes place in the
EIGRP process, but some settings are configured under the interface configuration
submode. This can add complexity for deployment and troubleshooting as users
must scroll back and forth between the EIGRP process and individual network
interfaces. Some of the settings that are set individually are hello advertisement
interval, split-horizon, authentication, and summary route advertisements.
Classic configuration requires the initialization of the routing process with the global
configuration command router eigrp as-number to identify the ASN and initialize the
EIGRP process. The second step is to identify the network interfaces with the
command network ip-address [mask].

13
EIGRP Named Mode
EIGRP named mode configuration was released to overcome some of the difficulties
network engineers have with classic EIGRP autonomous system configuration,
including scattered configurations and unclear scope of commands.
EIGRP named configuration provides the following benefits:
• All the EIGRP configuration occurs in one location.
• It supports current EIGRP features and future developments.
• It supports multiple address families (including Virtual Routing and Forwarding
[VRF] instances). EIGRP named configuration is also known as multi-address
family configuration mode.
• Commands are clear in terms of the scope of their configuration.

14
EIGRP Named Mode (Cont.)
EIGRP named configuration makes it possible to run multiple instances under the same EIGRP
process. EIGRP named mode provides a hierarchical configuration and stores settings in three
subsections:
• Address Family - This submode contains settings that are relevant to the global EIGRP AS
operations, such as selection of network interfaces, EIGRP K values, logging settings, and stub
settings.
• Interface - This submode contains settings that are relevant to the interface, such as hello
advertisement interval, split-horizon, authentication, and summary route advertisements. In
actuality, there are two methods of the EIGRP interface section’s configuration. Commands can be
assigned to a specific interface or to a default interface, in which case those settings are placed on
all EIGRP-enabled interfaces. If there is a conflict between the default interface and a specific
interface, the specific interface takes priority over the default interface.
• Topology - This submode contains settings regarding the EIGRP topology database and how
routes are presented to the router’s RIB. This section also contains route redistribution and
administrative distance settings.

15
EIGRP Network Statement
Both configuration modes use
a network statement to identify
the interfaces that EIGRP will
use. The network statement
uses a wildcard mask, which
allows the configuration to be
as specific or ambiguous as
necessary.
The syntax for the network
statement, which exists under
the EIGRP process, is
network ip-address [mask].
The optional mask can be
omitted to enable interfaces
that fall within the classful
boundaries for that network
statement.
To help illustrate the concept of the wildcard mask, Table 2-4 provides a set
of IP addresses and interfaces for a router.
The configuration in Example 2-1 enables EIGRP only on interfaces that explicitly
match the IP addresses in Table 2-4.

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Sample Topology and Configuration
Figure 2-5 shows a sample topology
for demonstrating EIGRP
configuration in classic mode for R1
and named mode for R2.
Example 2-3 provides the
configuration that is applied to R1 and
R2.

17
Confirming Interfaces
After configuration, it is a good
practice to verify that only the
intended interfaces are running
EIGRP. The command show ip eigrp
interfaces [{interface-id [detail] |
detail}] shows active EIGRP
interfaces.
Appending the optional detail keyword
provides additional information, such
as authentication, EIGRP timers, split
horizon, and various packet counts.
Example 2-5 demonstrates R1’s non-
detailed EIGRP interface and R2’s
detailed information for the gi0/1
interface.

18
Confirming Interfaces (Cont.)
Interface Interfaces running EIGRP.
Peers Number of peers detected on that interface.
Xmt Queue Un/Reliable Number of unreliable/reliable packets
remaining in the transmit queue. The value
zero is an indication of a stable network.
Mean SRTT Average time for a packet to be sent to a
neighbor and a reply from that neighbor to
be received, in milliseconds.
Multicast Flow Timer Maximum time (seconds) that the router
sent multicast packets
Pending Routes Number of routes in the transmit queue that
need to be sent.

19
Verifying EIGRP Neighbor Adjacencies
Each EIGRP process maintains a table of
neighbors to ensure that they are alive and
processing updates properly. Without
keeping track of a neighbor state, an
autonomous system could contain incorrect
data and could potentially route traffic
improperly. EIGRP must form a neighbor
relationship before a router advertises
update packets containing network prefixes.
The command show ip eigrp neighbors
[interface-id] displays the EIGRP neighbors
for a router. Example 2-6 shows the EIGRP
neighbor information using this command.
Field Description
Address IP address of the EIGRP neighbor
Interface Interface the neighbor was detected on
Holdtime Time left to receive a packet from this neighbor to ensure it is
still alive
SRTT Time for a packet to be sent to a neighbor and reply to be
received from that neighbor in milliseconds
RTO Timeout for retransmission (waiting for ACK)
Q cnt Number of packets (update/query/reply) in queue for sending
Seq Num Sequence number that was last received from the router

20
Displaying Installed EIGRP Routes
You can see EIGRP routes that are installed into
the RIB by using the command show ip route
eigrp.
• EIGRP routes originating within the
autonomous system have an administrative
distance (AD) of 90 and are indicated in the
routing table with a D.
• Routes that originate from outside the
autonomous system are external EIGRP
routes
• External EIGRP routes have an AD of 170
and are indicated in the routing table with D
EX.
• Placing external EIGRP routes into the RIB
with a higher AD acts as a loop-prevention
mechanism.
Example 2-7 displays the EIGRP routes from the
sample topology in Figure 2-5. The metric for the
selected route is the second number in brackets.

21
Router ID
• The router ID (RID) is a 32-bit
number that uniquely identifies an
EIGRP router and is used as a loop-
prevention mechanism.
• The RID can be set dynamically,
which is the default, or manually.
• The algorithm for dynamically
choosing the EIGRP RID uses the
highest IPv4 address of any up
loopback interfaces. If there are not
any up loopback interfaces, the
highest IPv4 address of any active up
physical interfaces becomes the RID
when the EIGRP process initializes.
You use the command eigrp router-id to set
the RID, as demonstrated in Example 2-8, for
both classic and named mode configurations.

22
Passive Interfaces
Some network topologies must advertise
a network segment into EIGRP but need
to prevent neighbors from forming
adjacencies with other routers on that
segment. In this scenario, you need to
put the EIGRP interface in a passive
state. Passive EIGRP interfaces do not
send out or process EIGRP hellos, which
prevents EIGRP from forming
adjacencies on that interface.
To configure an EIGRP interface as
passive, you use the command passive-
interface interface-id under the EIGRP
process for classic configuration.
Example 2-9 demonstrates making R1’s gi0/2
interface passive and also the alternative option
of making all interfaces passive but setting gi0/1
as non-passive.

23
Passive Routes (Cont.)
For a named mode configuration, you
place the passive-interface state on af-
interface default for all EIGRP
interfaces or on a specific interface with
the af-interface interface-id section.
Example 2-10 shows how to set the
gi0/2 interface as passive while allowing
the gi0/1 interface to be active using
both configuration strategies.
The command show ip protocols provides
valuable information about all the routing
protocols. With EIGRP, it displays the
EIGRP process identifier, the ASN, K values
that are used for path calculation, RID,
neighbors, AD settings, and all the passive
interfaces.
Example 2-11 shows what the named mode configuration
looks like with some settings (i.e. passive-interface or no
passive-interface) placed under the af-interface default or
the af-interface interface-id setting.

24
Authentication
• Authentication is a mechanism for ensuring that
only authorized routers are eligible to become
EIGRP neighbors.
• Authentication prevents adding a router to a
network and introducing invalid routes, accidentally
or maliciously.
• A precomputed password hash is included with all
EIGRP packets, and when the packet is received,
the receiving router also calculates the hash on the
packet. If the two hash values match, the packet
is accepted.
• EIGRP encrypts the password by using a Message
Digest 5 (MD5) authentication, using the keychain
function. The hash consists of the key number and
a password. EIGRP authentication does not
encrypt the contents of the routing update packets.
Keychain creation is accomplished
with the following steps:
Step 1. Create the keychain by
using the command key chain key-
chain-name.
Step 2. Identify the key sequence
by using the command key key-
number, where key-number can be
anything from 0 to 2147483647.
Step 3. Specify the preshared
password by using the command
key-string password.

25
Enabling Authentication on the Interface
When using classic configuration, authentication must be
enabled on the interface under the interface configuration
submode. The following commands are used in the
interface configuration submode:
• ip authentication key-chain eigrp as-number key-
chain-name
• ip authentication mode eigrp as-number md5
The named mode configuration places the configurations
under the EIGRP interface submode, under the af-interface
default or the af-interface interface-id. Named mode
configuration supports MD5 or Hashed Message
Authentication Code-Secure Hash Algorithm-256 (HMAC-
SHA-256) authentication. MD5 authentication involves the
following commands:
• authentication key-chain eigrp key-chain-name
• authentication mode md5
The HMAC-SHA-256 authentication involves the command
authentication mode hmacsha-256 password.
Example 2-14 demonstrates MD5
configuration on R1 with classic EIGRP
configuration and on R2 with named
mode configuration. Remember that the
hash is computed using the key
sequence number and key string, which
must match on the two nodes.

26
Verification of Keychain Settings
The command show key chain provides
verification of the keychain. Example 2-15
shows that each key sequence provides
the lifetime and password.
Example 2-16 provides detailed EIGRP
interface output.

27
Path Metric Calculations
• Metric calculation is a critical component for any routing protocol.
• EIGRP uses multiple factors to calculate the metric for a path.
• Metric calculation uses bandwidth and delay by default but can include interface load
and reliability, too.

28
Path Metric Calculation
EIRG Classic Metric Formula
The formula shown in Figure 2-6 illustrates
the EIGRP classic metric formula.
EIGRP uses K values to define which
factors the formula uses and the impact
associated with a factor when calculating
the metric. BW represents the slowest link in
the path, scaled to a 10 Gbps link (107
). Link
speed is collected from the configured
interface bandwidth on an interface. Delay is
the total measure of delay in the path,
measured in tens of microseconds (μs).

29
EIRG Classic Metric Formula (Cont.)
The EIGRP formula is based on the IGRP
metric formula, except the output is
multiplied by 256 to change the metric from
24 bits to 32 bits. Taking these definitions
into consideration, the formula for EIGRP
is shown in Figure 2-7.
By default, K1 and K3 have a value of
1, and K2, K4, and K5 are set to 0.
Figure 2-8 places default K values into
the formula and shows a streamlined
version of the formula.

30
EIGRP Attribute Propagation
The EIGRP update packet includes path attributes associated with each prefix. The EIGRP path
attributes can include hop count, cumulative delay, minimum bandwidth link speed, and RD. The
attributes are updated each hop along the way, allowing each router to independently identify the
shortest path.
Figure 2-9 shows the information in the EIGRP update packets for the 10.1.1.0/24 prefix
propagating through the autonomous system. Notice that as the hop count increments, minimum
bandwidth decreases, total delay increases, and the RD changes with each EIGRP update.

31
Default EIGRP Interface Metrics for Classic Metrics
Table 2-7 shows some of the
common network types, link speeds,
delay, and EIGRP metric, using the
streamlined formula from Figure 2-7.
If you are unsure of the
EIGRP metrics, you can
query the parameters for
the formula directly from
EIGRP’s topology table by
using the command show
ip eigrp topology network/
prefix-length.
Table 2-7 Default EIGRP Interface Metrics for Classic Metrics
Interface Type Link
Speed
(Kbps)
Delay Metric
Serial 64 20,000 μs 40,512,000
T1 1544 20,000 μs 2,170,031
Ethernet 10,000 1000 μs 281,600
Fast Ethernet 100,000 100 μs 28,160
GigabitEthernet 1,000,000 10 μs 2816
TenGigabitEthernet 10,000,00
0
10 μs 512

32
Wide Metrics
Example 2-18 provides some metric
calculations for common LAN interface
speeds. Notice that there is not a
differentiation between an 11 Gbps interface
and a 20 Gbps interface. The composite
metric stays at 256, despite the different
bandwidth rates.
EIGRP includes support for a second set of
metrics, known as wide metrics, that
addresses the issue of scalability with
higher-capacity interfaces.
Figure 2-11 shows the explicit EIGRP wide metrics
formula. Notice that an additional K value (K6) is
included that adds an extended attribute to measure
jitter, energy, or other future attributes.

33
Wide Metrics (Cont.)
Just as EIGRP scaled by 256 to
accommodate IGRP, EIGRP wide metrics
scale by 65,535 to accommodate higher-
speed links. This provides support for
interface speeds up to 655 terabits per
second (65,535 × 107) without any
scalability issues.
Latency is the total interface delay
measured in picoseconds (10−12
) instead
of in microseconds (10−6
). Figure 2-12
shows an updated formula that takes into
account the conversions in latency and
scalability.
The EIGRP classic metrics exist only with
EIGRP classic configuration, while EIGRP wide
metrics exist only in EIGRP named mode. The
metric style used by a router is identified with
the command show ip protocols; if a K6 metric
is present, the router is using wide-style metrics.

34
Metric Backward Compatibility
EIGRP wide metrics were designed with
backward compatibility in mind. EIGRP
wide metrics set K2 and K3 to a value of 1
and set K2, K4, K5, and K6 to 0, which
allows backward compatibility because
the K value metrics match with classic
metrics. As long as K1 through K5 are the
same and K6 is not set, the two metric
styles allow adjacency between routers.
EIGRP is able to detect when peering
with a router is using classic metrics, and
it unscales the metric to the formula in
Figure 2-13.
This conversion results in loss of clarity if routes
pass through a mixture of classic metric and
wide metric devices. It is best to keep all
devices operating with the same metric style.

35
Interface Delay Settings
Example 2-20 provides sample output of
the command on R1 and R2. Both
interfaces have a delay of 10.
EIGRP delay is set on an interface-by-interface basis,
allowing for manipulation of traffic patterns flowing
through a specific interface on a router. Delay is
configured with the interface parameter command
delay tens-of-microseconds under the interface.
Example 2-21 demonstrates the modification of the
delay on R1 to 100, increasing the delay to 1000 μs
on the link between R1 and R2. To ensure consistent
routing, modify the delay on R2’s gi0/1 interface as
well, then verify the change.

36
Custom K Values
If the default metric calculations are insufficient, you can change them to modify the path
metric formula.
• K values for the path metric formula are set with the command metric weights TOS K1
K2 K3 K4 K5 [K6] under the EIGRP process.
• The TOS value always has a value of 0, and the K6 value is used for named mode
configurations.
• To ensure consistent routing logic in an EIGRP autonomous system, the K values must
match between EIGRP neighbors to form an adjacency and exchange routes.
• The K values are included as part of the EIGRP hello packet.
The K values are displayed with the show ip protocols command.

37
Load Balancing
EIGRP allows multiple successor routes
(with the same metric) to be installed into
the RIB. Installing multiple paths into the
RIB for the same prefix is called equal-cost
multipathing (ECMP) routing. The default
maximum ECMP is four routes. You change
the default ECMP setting with the command
maximum-paths under the EIGRP process
in classic mode and under the topology
base submode in named mode.
Example 2-22 shows the configuration for
changing the maximum paths on R1 and R2
so that classic and named mode
configurations are visible.

38
Load Balancing (Cont.)
EIGRP supports unequal-cost load
balancing, which allows installation of
both successor routes and feasible
successors into the EIGRP RIB. To use
unequal-cost load balancing with
EIGRP, change EIGRP’s variance
multiplier. The EIGRP variance value is
the feasible distance (FD) for a route
multiplied by the EIGRP variance
multiplier. Any feasible successor’s FD
with a metric below the EIGRP variance
value is installed into the RIB. Dividing the feasible successor metric by the
successor route metric provides the variance multiplier.
The variance multiplier is a whole number, and any
remainders should always round up.

39
Prepare for the Exam

40
Key Topics for Chapter 2
Description
EIGRP terminology Authentication
Topology table Path metric calculation
EIGRP packet types EIGRP attribute propagation
Forming EIGRP neighbors EIGRP wide metrics formula
Classic configuration mode Custom K values
EIGRP named mode Unequal-cost load balancing
Passive interfaces

41
Key Terms for Chapter 2
Term
autonomous system (AS) topology table
successor route EIGRP classic configuration
successor EIGRP named mode configuration
feasible distance passive interface
reported distance K values
feasibility condition wide metrics
feasible successor variance value

42
Command Reference for Chapter 2
Task Command Syntax
Initialize EIGRP in classic
configuration
router eigrp as-number
network network mask
Initialize EIGRP in named
mode configuration
router eigrp process-name address-family { ipv4 | ipv6 } {unicast |
vrf vrf-name} autonomous-system as-number network network-
mask
Define the EIGRP router ID eigrp router-id router-id
Configure an EIGRP-enabled
interface to prevent neighbor
adjacencies
Classic: passive-interface interface-id
Named Mode: af-interface {default | interface-id}
passive-interface
Configure a keychain for
EIGRP MD5 authentication
key chain key-chain-name
key key-number
key-string password

43
Command Reference for Chapter 2 (Cont.)
Task Command Syntax
Configure MD5 authentication for an EIGRP
interface
Classic: (EIGRP Process)
ip authentication key-chain eigrp as-number key-chain-
name
ip authentication mode eigrp as-number md5
authentication key-chain eigrp key-chain-name
authentication mode md5
Configure SHA authentication for EIGRP
named mode interfaces
authentication mode hmac-sha-256 password
Modify the interface delay for an interface delay tens-of-microseconds
Modify the EIGRP K values metric weights TOS K1 K2 K3 K4 K5 [K6]
Modify the default number of EIGRP
maximum paths that can be installed into the
RIB
Maximum-paths maximum-paths

44
Command Reference for Chapter 2 (Cont.)
Task Command Syntax
Modify the EIGRP variance multiplier for
unequal-cost load balancing
variance multiplier
Display the EIGRP-enabled interfaces show ip eigrp interface [{interface-id [detail] |
detail}]
Display the EIGRP topology table show ip eigrp topology [all-links]
Display the configured EIGRP
keychains and passwords
show key chain
Display the IP routing protocol
information configured on the router
show ip protocols

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