08 - Routing - EIGRP

Routing Protocols and Concepts

EIGRP

Chapter 08

Objectives

� Describe the background and history of Enhanced

Interior Gateway Routing Protocol (EIGRP).

� Examine the basic EIGRP configuration commands and identify their purposes.

� Calculate the composite metric used by EIGRP.

� Describe the concepts and operation of DUAL.

� Describe the uses of additional configuration commands in EIGRP.

Introduction

Terms you MUST know

RIPng

Weights

Hold time

TLV

Neighbor Table

Routing Table

Topology Table

Bounded Updates

Summary routes

Quan zeros

AS

Wildcard mask

IPv6

K values

Reliability

Load

Delay

Bandwidth

Successor

Feasible Distance

RD

FC

Feasible Successor

FSM

Internal routes

External routes

OP Code

PDM

RTP

Unicast

Multicast

Hello packet

Adjacencies

DUAL

IANA

Null 0

Aperiodic

EIGRP

� Roots of EIGRP: IGRP

� -Developed in 1985 to overcome RIPv1’s limited hop count

� -Distance vector routing protocol

� -Metrics used by IGRP

� bandwidth (used by default)

� Delay (used by default)

� reliability

� load

� -IGRP discontinued support starting with IOS 12.2(13)T & 12.2(R1s4)S

EIGRP

EIGRP Message Format

� EIGRP Header

� Data link frame header - contains source and destination MAC address

� IP packet header - contains source & destination IP address

� EIGRP packet header - contains AS number

� Type/Length/Field - data portion of EIGRP message

EIGRP

� EIGRP packet headercontains

� Opcode field

� Autonomous System number

� EIGRP Parameters contains

� Weights

� Hold time

EIGRP

� TLV: IP internal contains

� Metric field

� Subnet mask field

� Destination field

� TLV: IP external contains

� Fields used when external

� routes are imported into

� EIGRP routing process

EIGRP

Protocol Dependent Modules (PDM)

� EIGRP uses PDM to route

several different protocols i.e.

IP, IPX & AppleTalk

� PDMs are responsible for the

specific routing task for each

network layer protocol

EIGRP

Reliable Transport Protocol (RTP)

� Purpose of RTP

� Used by EIGRP to transmit and receive

EIGRP packets

� Characteristics of RTP

� Involves both reliable & unreliable delivery

of EIGRP packet

� Reliable delivery requires acknowledgment

from destination

� Unreliable delivery does not require an

acknowledgement from destination

� Packets can be sent

� Unicast

� Multicast

� Using address 224.0.0.10

EIGRP

EIGRP’s 5 Packet Types

� Hello packets

� Used to discover & form adjacencies with neighbors

EIGRP

� Update packets

� Used to propagate routing information

� Acknowledgement packets

� Used to acknowledge receipt of update, query & reply packets

EIGRP

� Query & Reply packets

� Used by DUAL for searching for networks

� Query packets

� -Can use

� Unicast

� Multicast

� Reply packet

� -Use only

� unicast

EIGRP

� Purpose of Hello Protocol

� To discover & establish adjacencies with neighbor routers

� Characteristics of hello protocol� Time interval for sending hello packet

� Most networks it is every 5 seconds

� Multipoint non broadcast multi-access networks� Unicast every 60 seconds

-Holdtime

�This is the maximum time router should wait before declaring a neighbor down

�Default holdtime

–3 times hello interval

EIGRP

EIGRP Bounded Updates

� EIGRP only sends update when there is a change inroute status

� Partial update� A partial update includes only the route information that has

changed – the whole routing table is NOT sent

� Bounded update� When a route changes, only those devices that are impacted

will be notified of the change

� EIGRP’s use of partial bounded updates minimizes use of bandwidth

EIGRP

Diffusing Update Algorithm (DUAL)

� Purpose

� EIGRP’s primary method for preventing routing loops

� Advantage of using DUAL

� Provides for fast convergence time by keeping a list of loop-

free backup routes

EIGRP

� Administrative Distance (AD)

� Defined as the trustworthiness of the source route

� EIGRP default administrative distances

� Summary routes = 5

� Internal routes = 90

� Imported routes = 170

EIGRP

Authentication

� EIGRP can

� Encrypt routing information

� Authenticate routing information

EIGRP

Network Topology

� Topology used is the same as previous chapters with

the addition of an ISP router

EIGRP

� EIGRP will automatically

summarize routes at classful boundaries

EIGRP

Basic EIGRP Configuration

� Autonomous System (AS) & Process IDs

� This is a collection of networks under the control of a single authority (reference RFC 1930)

� AS Numbers are assigned by IANA

� Entities needing AS numbers

� ISP

� Internet Backbone prodiers

� Institutions connecting to other institutions using AS numbers


� EIGRP autonomous system number actually

functions as a process ID

� Process ID represents an

instance of the routing

protocol running on a router

� Example

Router(config)#router

eigrp autonomous-system


The router eigrp command

� The global command that enables eigrp is

� router eigrp autonomous-system

� -All routers in the EIGRP routing domain must use

the same process ID number (autonomous-system

number)


The Network Command

� Functions of the network

command

� Enables interfaces to transmit & receive EIGRP updates

� Includes network or subnet in EIGRP updates

� Example

� Router(config-router)#network network-address


� The network Command with a Wildcard Mask

� -This option is used when you want to configure

EIGRP to advertise specific subnets

� -Example

� Router(config-router)#network network-address [wildcard-mask]


Verifying EIGRP

� EIGRP routers must establish adjacencies with their

neighbors before any updates can be sent or received

� Command used to view neighbor table and verify that

EIGRP has established adjacencies with neighbors is

� show ip eigrp neighbors

EIGRP

� The show ip protocolscommand is also used

to verify that EIGRP is

enabled


Examining the Routing Table

� The show ip routecommand is also used to verify EIGRP

� EIGRP routes are denoted in a routing table by the letter “D”

� By default , EIGRP automatically summarizes routes at major network boundary


� Introducing the Null0 Summary Route

� Null0 is not a physical interface

� In the routing table summary routes are sourced from Null0

� Reason: routes are used for advertisement purposes

� EIGRP will automatically include a null0 summary route as child route when 2 conditions are met

� At least one subnet is learned via EIGRP

� Automatic summarization is enabled


� R3’s routing table

shows that the 172.16.0.0/16 network

is automatically summarized by R1 &

R3

EIGRP Metric Calculation

EIGRP Composite Metric & the K Values� EIGRP uses the following values in its composite

metric� -Bandwidth, delay, reliability, and load

� The composite metric used by EIGRP� formula used has values K1 �K5

� K1 & K3 = 1� all other K values = 0


� Use the sh ip protocols command to verify the K values


EIGRP Metrics

� Use the show interfaces command

to view metrics

� EIGRP Metrics

� Bandwidth – EIGRP uses a static bandwidth to calculate metric

� Most serial interfaces use a default bandwidth value of 1.544Mbos (T1)


EIGRP Metrics

� Delay is defined as

the measure of time it takes for a packet

to traverse a route

� -it is a static value

based on link type to which

interface is

connected


� Reliability (not a default EIGRP metric)� -A measure of the likelihood that a link will fail

� -Measure dynamically & expressed as a fraction of 255

� the higher the fraction the better the reliability

� Load (not a default EIGRP metric)� A number that reflects how much traffic is using a link

� Number is determined dynamically and is expressed as a fraction of 255� The lower the fraction the less the load on the link


Using the Bandwidth Command

� Modifying the interface

bandwidth

� -Use the bandwidth command

� -Example

� Router(config-if)#bandwidthkilobits

� Verifying bandwidth

� Use the show interfacecommand

� Note – bandwidth command does not change thelink’s physical bandwidth


� The EIGRP metric can be determined by examining the

bandwidth delay


� EIGRP uses the lowest bandwidth (BW)in its metric

calculation

Calculated BW = reference BW / lowest BW(kbps)

� Delay – EIGRP uses the cumulative sum of all outgoing

interfaces

� Calculated Delay = the sum of outgoing interface

delays

� EIGRP Metric = calculated BW + calculated delay




DUAL Concepts

� The Diffusing Update Algorithm (DUAL) is used to prevent looping

DUAL Concepts

� Successor

� The best least cost route to a

destination found

in the routing table

� Feasible distance

� The lowest

calculated metric

along a path to a destination

network

DUAL Concepts

Feasible Successors, Feasibility Condition & Reported Distance

� Feasible Successor

-This is a loop free backup route to same

destination as successor route

DUAL Concepts

� Reported distance (RD)

� -The metric that

a router reports to a neighbor

about its own

cost to that

network

Feasible Successors, Feasibility Condition & Reported Distance

DUAL Concepts

� Feasibility Condition (FC)

� -Met when a neighbor’s RD

is less than the local router’s

FD to the same

destination network

DUAL Concepts

� EIGRP

Topology

Table

dissected

DUAL Concepts

Topology Table: No Feasible Successor

� A feasible successor may not be present because the

feasibility condition may not be met

� -In other words, the reported distance of the

neighbor is greater than or equal to the current

feasible distance

DUAL Concepts

DUAL Concepts

� Finite Sate Machine (FSM)

� An abstract machine that defines a set of possible

states something can go through, what event causes those states and what events result form

those states

� FSMs are used to describe how a device,

computer program, or routing algorithm will react

to a set of input events

DUAL Concepts

� DUAL FSM

� Selects a best

loop-free path to a destination

� Selects alternate routes by using

information in EIGRP tables

DUAL Concepts

Finite State Machines (FSM)

� To examine output from EIGRP’s finite state machine us the debug eigrp fsm command

More EIGRP Configurations

The Null0 Summary Route

� By default, EIGRP uses the Null0 interface to discard

any packets that match the parent route but do not match any of the child routes

� EIGRP automatically includes a null0 summary route as a child route whenever both of the following

conditions exist

� One or subnets exists that was learned via EIGRP

� Automatic summarization is enabled


The Null0 Summary Route


Disabling Automatic Summarization

� The auto-summary command permits EIGRP to automatically summarize at major network boundaries

� The no auto-summary command is used to disable automatic summarization

� This causes all EIGRP neighbors to send updates that will not be automatically summarized� this will cause changes to appear in both

� -routing tables

� -topology tables


Manual Summarization

� Manual summarization can include supernets

� Reason: EIGRP is a classless routing protocol & include subnet mask in update

� Command used to configure manual summarization

� Router(config-if)#ip summary-address eigrp as-number network-address subnet-mask


� Configuring a summary route in EIGRP


EIGRP Default Routes

� “quad zero” static default route

� -Can be used with any currently supported routing

protocol

� -Is usually configured on a router that is connected a network outside the EIGRP domain

� EIGRP & the “Quad zero” static default route

� Requires the use of the redistribute static

command to disseminate default route in EIGRP updates


Fine-Tuning EIGRP

� EIGRP bandwidth utilization

� -By default, EIGRP uses only up to 50% of interface bandwidth for EIGRP information

� -The command to change the percentage of bandwidth used by EIGRP is

Router(config-if)#ip bandwidth-percent eigrp as-number percent


� Configuring Hello Intervals and Hold Times� -Hello intervals and hold times are configurable on a per-interface

basis

� -The command to configure hello interval is

� Router(config-if)#ip hello-interval eigrp as-number seconds

� Changing the hello interval also requires changing the

hold time to a value greater than or equal to the hello

interval

� -The command to configure hold time value is

� Router(config-if)#ip hold-time eigrp as-number seconds

Summary

� Background & History

� EIGRP is a derivative of IGRP

� EIGRP is a Cisco proprietary distance vector routing protocol

released in 1994

� EIGRP terms and characteristics

� EIGPR uses RTP to transmit & receive EIGRP packets

� EIGRP has 5 packet type:

� Hello packets

� Update packets

� Acknowledgement packets

� Query packets

� Reply packets

� Supports VLSM & CIDR

Summary

� EIGRP terms and characteristics

� EIGRP uses a hello protocol

� Purpose of hello protocol is to discover & establish adjacencies

� EIGRP routing updates

� Aperiodic

� Partial and bounded

� Fast convergence

Summary

� EIGRP commands� The following commands are used for EIGRP

configuration� RtrA(config)#router eigrp [autonomous-system #]

� RtrA(config-router)#network network-number

� The following commands can be used to verify EIGRP� Show ip protocols

� Show ip eigrp neighbors

� Show ip route

Summary

� EIGRP metrics include

� Bandwidth (default)

� Delay (default)

� Reliability

� Load

Summary

� DUAL� Purpose of DUAL

� To prevent routing loops

� Successor� Primary route to a destination

� Feasible successor� Backup route to a destination

� Feasible distance� Lowest calculated metric to a destination

� Reported distance� The distance towards a destination as advertised by an

upstream neighbor

Summary

� Choosing the best route

� After router has received all updates from directly

connected neighbors, it can calculate its DUAL

� 1st metric is calculated for each route

� 2nd route with lowest metric is designated successor & is placed in routing table

� 3rd feasible successor is found

� Criteria for feasible successor: it must have lower reported

distance to the destination than the installed route’s feasible

distance

� Feasible routes are maintained in topology table

Summary

� Automatic summarization

� On by default

� Summarizes routes on classful boundary

� Summarization can be disabled using the

following command

� RtrA(config-if)#no auto-summary

EIGRP – some more

EIGRP

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

EIGRP supports CIDR, and hence VLSM.

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

Technically, EIGRP is an advanced distance-vector routing protocolthat relies on features commonly associated with link-state protocols. Some of OSPF’s best traits, such as partial updates & neighbor discovery, are similarly put to use by EIGRP.

EIGRP

Recall that OSPF’s strengths include scalability and multi-vendor support. So if your core routers are a mixed bag of products from several different vendors, OSPF and RIPmay be your only options.

But OSPF’s benefits, especially it’s hierarchical design, come at a price: administrative complexity.

EIGRP is an ideal choice for large, multiprotocol networks built primarily on Cisco routers (says Cisco).

Comparison of OSPF and EIGRP

OSPF EIGRP

Supports CIDR & VLSM, rapid convergence, partial updates, neighbor discovery.

Supports CIDR & VLSM, rapid convergence, partial updates, neighbor discovery

Enables the admin to define route summarization.

Uses automatic route summarization & user-defined route summaries.

Is an open standard; has multi-vendor support

Is proprietary; can only be used with Cisco routers.

Is scalable; admin defined ‘areas’ provide manageable hierarchy.

Is scalable; no hierarchical domains exist

Is difficult to implement Is easy to implement.

EIGRP and IGRP Compatibility

EIGRP offers multiprotocol support and IGRP does not.

EIGRP scales IGRP’s metric by a factor of 256. That’s because EIGRP uses a metric that is 32 bits long, & IGRPuses a 24-bit metric. By dividing or multiplying by 256, EIGRP can easily exchange information with IGRP.

EIGRP imposes a max hop limit of 224, where IGRP has 255.

Sharing or redistribution, is automatic between IGRP & EIGRP as long as both processes use the sameAS number.

metric = [K1 x bandwidth + K2 x bandwidth) / (256ms – load) + (K3 x delay)] x

[K5 / (reliability+K4)]

Where by default: K1 = 1 K2 = 0 K3 = 1 K4 = 0 K5 = 0

EIGRP and IGRP Compatibility

EIGRP tags routes learned from IGRP as externalbecause they did not originate from EIGRP routers. External EIGRP routes are denoted by EX in the routing table.

But IGRP cannot differentiate between internal & external routes

EIGRP vs. IGRP

EIGRP

EIGRP is an advanced distance vector routing protocol, but has advantages over simple distance vector protocols:

• rapid convergence

By using an routing algorithm, Diffusing Update Algorithm (DUAL)which guarantees loop-free operation & allows all routers involved in a topology change to synchronize at the same time.

• partial bounded updates

EIGRP routers make partial, incremental updates, & unlike OSPF, the routers send these partial updates only to the routers that need the information. This is called bounded updates.

EIGRP cont.

• minimal consumption of bandwidth when the network is stable

•No timed routing updates - instead small hello packets. •Exchanged a regular intervals •don’t use a significant amount of bandwidth.

• support for VLSM and CIDR

• multiple network-layer support

EIGRP supports IP, IPX and AppleTalk via protocol-dependent modules (PDM).

• complete independence from routed protocols

PDM protect EIGRP from painstaking revision.

EIGRP Terminology & Tables

EIGRP routers keep route and topology information in RAM for quick access.

EIGRP terms and tables:

neighbor table

Maintains a neighbor table that list adjacent routers. There is a neighbor table for each protocol that EIGRP supports.

topology table

Topology table for each configured network protocol. All learned routes to a destination are maintained in the topology table.

routing table

EIGRP chooses the best (successor) routes to a destination from the topology table & places these routes in the routing table.

successora route selected as the primary route used to reach a destination. Successors are kept in the routing table.

feasible successorIs a backup route. They are kept in the topology table.

EIGRP routers establish adjacencies with neighbor routers by sending hello packets, sent every 5 seconds (default).

By forming adjacencies, EIGRP routers do the following:

• dynamically learn of new routes that join their network

• identify routers that become either unreachable or inoperable

• rediscover routers that had previously been unreachable.

EIGRP Terminology

EIGRP Technologies cont.

Reliable Transport Protocol (RTP) is a transport layer (layer 4) protocol that can guarantee ordered delivery of EIGRP packets to all neighbors. To stay independent of IP, EIGRP uses its own proprietary transport-layer protocol to guarantee delivery of routing information.

EIGRP uses RTP to provide reliable or unreliable service as the situation warrants. Hello packets are not required to be reliable delivery. RTP supports both unicasting & multicasting , and it can multicast and unicast to different peers simultaneously.

The centerpiece of EIGRP is DUAL, the EIGRP route calculation engine. It uses a finite state machine.

DUAL tracks all the routes advertised by neighbors and uses the composite metric of each route to compare them. DUAL also guarantees that each path is loop-free.

EIGRP Technologies cont.

A successor is a neighboring router that is currently used for packet forwarding; it provides the least cost route to the destination and is not part of the routing loop

A feasible successor provides the next lowest cost path without introducing routing loops.

EIGRP Data Structure

Neighbor Table

Routing Table

Topology Table

Reported distance (RD)

Feasible distance (FD)


Neighbor Table

the most important table in EIGRP. The neighbor relationships in the neighbor table are basis for all EIGRP routing updates and convergence activity. supports reliable, sequenced delivery of packets.

Routing Table

Contain the routes installed by DUAL as the best loop free paths to a given destination. It canmaintain up to 4 routesper destination.

Topology Table

stores all the information it needs to calculate a set of distances and vectors to all reachable destinations.


Reported distance (RD)

The distance reported by an adjacent neighbor to a specific destination.

Feasible distance (FD)

The lowest calculated metric to each destination.

Table is sorted with the successor routes at the top, followed by feasible successors.

At the bottom are what DUAL believes are routing loops.

To see the topology table, use the command:

Router# show ip eigrp topology [all]

Example of output:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33

via 10.2.0.2 (2681856/2169856), Serial1

Viewing (2681856/2169856), 2681856 is the FD, and 2169856 is the RD.

If no feasible successors to the destination exist, DUAL places the route in the active state.

Entries in the topology table can be in one of 2 states

• passive route – route that is stable and available for use

• active route – route in the process of being recomputed by DUAL.

Internal routes originate from within the EIGRP AS.

External routes originate from outside the system.

Routes learned (redistributed) from other routing protocols such as RIP, OSPF and IGRP are external.

Static routes originating from outside the EIGRP AS and redistributed inside are also external routes.

NOTE:

The internal administrative distance of EIGRP is 90.

EIGRP Packet Types

EIGRP relies on 5 packet types to maintain its various tables and establish complex relationships with neighbor routers.

The 5 packet types:

1. Hello

2. Acknowledgment

3. Update

4. Query

5. Reply

Hello Packets

EIGRP relies on hello packets to discover, verify & rediscover neighbor routers. The default hello interval depends on the bandwidth of the

interface:

Bandwidth Example Link Default Hello Default Hold

Interval Time

Less than 1.544Mbps Multipoint Frame Relay 60 sec 180 sec

Greater than 1.544Mbps T1, Ethernet 5 sec 15 sec

Keep the hold timer 3 times the hello interval. EIGRP hello packets are multicast. On IP networks,

EIGRP routers send hellos to the multicasts IP address 224.0.0.10.

Recall that OSPF requires neighbor routers to have the same hello & dead intervals to communicate. EIGRP has no such restriction.

Acknowledgement Packets

An EIGRP router uses acknowledgement packets to indicate receipt of any EIGRP packet during a reliable exchange.

To be reliable, a sender’s message must be acknowledged by the recipient.

Hello packets are always sent unreliable, & require no acknowledgement.

Update Packets

Update packets are used when a router discovers a new neighbor. They are also used when a router detects a topology change.

All update packets are sent reliably.

Query and Reply Packets

EIGRP routers use query packets whenever they need specific information from one or all of its neighbors. A reply packet is used to respond to a query.

Query can be multicast or unicast

Replies are always unicast.

Both packet types are sent reliably.

EIGRP Convergence (DUAL)

DUAL’s sophisticated algorithm results in EIGRP’sexceptional fast convergence, says Cisco

A router’s topology table includes a list of all routes advertised by the neighbors. For each network, the router keeps the real (computed) cost of getting to that network & also keeps the advertised cost (reported distance) from its neighbor.

The best path is the path with the lowest metric route that is calculated by adding the metric between the next-hop router and the destination (the reported distance) to the metric between the local router and the next-hop router.

This computed cost, or distance is the FD.

The next-hop router(s) are selected as the best path is the successor. If several routes have the same FD, then there can be several successors to a destination.

EIGRP Convergence (DUAL)

RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Report Distance to 24

RTX 40 30

RTY 31 21

RTZ 230 220

10

For RTA, RTY is the successor to Network 24, because it has the lowest computed cost (31). Hence RTA’s FD=31.

If RTY goes down, then is there a feasible successor – or not? If so, what is it?

RTA reaches Network 24 via:

RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Report Distance to 24

RTX 40 30

RTY 31 21

RTZ 230 220

10

Feasible successor =

1. It’s RD < RTA’s FD AND

2. Has the lowest computed cost of all other routesDUAL calculates the feasible successor to be through RTX

Remember: RTA’s FD=31

RTA

RTX

RTZ

FDDI

Network24

RTY

COST

Serial10

100 100

10

20

1

Neighbor Computed Cost to 24 Reported Distance to 24

RTX 40 30

RTY 31 21

RTZ 230 220

10

Feasible successor =

1. It’s RD < RTA’s FD AND

2. Has the lowest computed cost of all other routes

Suppose RTX also goes down. Is there a feasible successor for RTA to Network 24?

NO! DUAL will set the route to Network 24 from passive to active state, and RTA will query its neighbor’s about Network 24 again.

Consider the output from the command ‘show ip eigrptopology all’:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33via 10.2.0.2 (2681856/2169856), Serial0via 10.2.1.2 (2681856/2169856), Serial1via 10.2.2.2 (2891856/2769856), Ethernet 0

Remember:Successor is the route that has the best lowest cost or FD.Feasible successor =

It’s RD < link’s FD AND has the lowest computed costof all other routes

Consider the output from ‘show ip eigrp topology all’:

P 10.2.0.0/16, 1 successors, FD is 2681856, serno 33via 10.2.0.2 (2681856/2169856), Serial0via 10.2.1.2 (2681856/2169856), Serial1via 10.2.2.2 (2891856/2769856), Ethernet 0

What are the successor(s)?

10.2.0.2 AND 10.2.1.2

What are the feasible successor(s)?

NONE

EIGRP Operation

The 4 key steps to EIGRP operation are:

1. building the neighbor table

2. discovering routes

3. choosing routes

4. maintaining routes

Choose the routes based on these 5 factors:

1. bandwidth

2. delay

3. reliability

4. load

5. MTU (maximum transmission unit)

So, unless otherwise configured by an administrator, bandwidth & delay are the only 2 factors that determine EIGRP’s metric value.

Configuring EIGRP

Configuring EIGRP for IP Networks

Router(config)# router eigrp autonomous-system-

number

Router(config-router)# network network-number

network-number is the NETWORK address of the

interface of the router

Configuring EIGRP

The ip bandwidth-percent command configures the percentage of bandwidth that EIGRP can use on an

interface.

By default, EIGRP is set to use up to 50 percent of the

bandwidth of an interface to exchange routing information.

The command relies on the bandwidth of an interface.

Some cases the engineer sets the bandwidth to a lower

number than the actual bandwidth of the link (in order to

manipulate the routing metric).

Summarizing EIGRP routes for IP

EIGRP automatically summarizes routes at a classful boundary.

For discontiguous subnetworks, you do NOTwant summarization.

The command that turns summarization off on EIGRP is:

Router(config-router)# no auto-summary

Hence, with summarization turned off, EIGRP routers will advertise subnets.

Manual summary routes are configured on a per-interface basis by:

Router(config-if)#ip summary-address eigrpautonomous-system-number ip-address mask administrative-distance

By default, EIGRP summary routes have an administrative distance of 5. The value can range between 1 and 255.

Summarizing EIGRP routes for IP

END

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08 - Routing - EIGRP