eigrp open university

8/3/2019 EIGRP Open University

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EIGRP

Explanation & Configuration

By Bill Reed


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By Bill Reed

We WillExamine the features of EIGRP

Discuss why EIGRP is known as a Hybrid Routing

Protocol

Identify the way EIGRP Routers establishrelationships with neighbours using the DUAL

algorithm, and then communicate with them.

Describe the configuration of EIGRP.


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By Bill Reed EIGRP Features

Uses Multicasts for update traffic

Maintains databases similar to OSPF

Supports VLSM and CIDR Very short convergence time

Loop-Free topology generation

Support for multiple network protocols


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By Bill Reed EIGRP why Hybrid?

EIGRP is called a Hybrid Routing Protocol because itfeatures some of the characteristics of both DistanceVector (DV) and Link State (LS) Routing Protocols.

EIGRP Uses Metric Calculation like distance Vector (DV)

EIGRP does not send its entire Routing Table in its updates, itsends only the changes like Link State Protocols (LS)

EIGRP only sends updates when a change occurs within thenetwork topology, like Link State Protocols (LS)


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By Bill Reed First some basicTerminology

How EIGRP works


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By Bill Reed And somepacket types

How EIGRP works

And finallythe tables


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By Bill Reed

EIGRP operation is primarily based upon the

DUAL finite state machine

EIGRP operates by carrying out:

Neighbour Discovery

Route Discovery

Route Selection

Route Maintenance (DUAL fsmlives here)

A wordabout DUAL

How EIGRP works


8/19

By Bill Reed How EIGRP works Neighbour Discovery

Hello Packet

TOM

DICK

MARY

Hello = 60 Seconds for slow links

Hello = 5 Seconds for all other links


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Hello Packet

TOM

DICK

MARY

Hello = 60 Seconds for slow links

Hello = 5 Seconds for all other links


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TOM

Neighbour Table

Topology Table

Routing Table

Hello packet sentMULTICAST

UNRELIABLY

224.0.0.10

Hello packet received

AS & K valuesexamined

IF AS & K values match

Neighbour added


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TOM

Neighbour Table

Topology Table

Routing Table

MARY

Once both neighbours have identifiedone another and matched both ASand K values.

AnAdjacency has been formed

MARY


12/19

By Bill Reed How EIGRP works Route Discovery

TOM

Neighbour Table

Topology Table

Routing Table

MARY

Adjacent routers exchange routing

updates by means of

UPDATE PACKETS

MARY

U

Routing Updates are sent using RTP

Reliable Transport Protocol which is aCisco proprietary protocol.

Updates can contain multiple routesAnd can be advertising

internal or external routes


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INTERNAL ROUTES

Internal routes are defined as:

Those routes that originate within the EIGRP autonomous system

Internal route updates contain the following information:

Next hopDelayBandwidthMTUHop countReliabilityLoad

Prefix lengthDestination

The calculation of the metrics looks like this:

(K2 x (Be x 256))/(256 load) + K3 x (Dc x 256) x (K5/(reliability + K4))

[Be and Dc are multiplied by 256]

K1 = Bandwidth Weight

K2 = Load weightK3 = Delay weightK4 = Secondary reliability weightK5 = Primary reliability weightBe = Bandwidth component *Dc = Delay component *


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EXTERNAL ROUTES

External routes are defined as:

Those routes that originate outside of the EIGRP autonomous system

External route updates contain the following information:

Originating routerOrigination AS numberExternal protocol metricExternal protocol ID

Can be:

IGRP, EIGRP, Static routes, RIP, OSPF,IS-IS, EGP, BGP, IRDP or Directly connected

Updates are pre configured to use no more than 50% of the bandwidth.

This setting can be modified.

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TOM

Neighbour Table

Topology Table

Routing Table

DICK

MARY

Once updates have been exchanged

The Topology table can be compiledTo reflect the network.

Network a#

Network b#

MARY

B Bill R d


16/19

By Bill Reed How EIGRP works Route Selection

ROUTE SELECTION

For a route to be added to the routing table

it needs to meet the FC or feasibility condition

The feasibility condition states that:the advertised metric from the neighbour routermust be less than the FD feasible distance

on the local router for the route in question.

Using this methodology removes any possibility or routing loops

Only the best route (to the successor router) is added to the table

Any other routes that meet the FC are designated (feasible successors)

Routing Table

Route #1

Route 2#

B Bill R d


17/19

By Bill Reed How EIGRP works Route Maintenance

ROUTE MAINTENANCE (The DUAL Finite State Machine)

DUAL is the Algorythm that drives therouting topology identification and best path

functionality within EIGRP.

DUAL works like this:

Neighbour adjacencies are established (previously described) Neighbour failures are detected by lack of hello packets (limited update sent) Entire routing table sent when first made adjacent

Input events trigger local computations (changes & link failures) Diffuse computations occur when local computations fail to resolve Whenever a change occurs in the routing table, all neighbours are informed.

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By Bill Reed How EIGRP works Route Summarisation

ROUTE SUMMARISATION

EIGRP includes subnet mask information in its updates.

This allows EIGRP to support discontiguous networks, VLSM and CIDR.

EIGRP can also perform manual summarisation.

Things to remember!

Auto summarisation is enabled by default on an EIGRP router By invoking manual summarisation using subnets more flexibly is possible turning off auto summarisation can dramatically enlarge routing tables

Large routing tables eat into memory and CPU cycles on an EIGRP router If a route which is part of a summary is unstable

it has the potential to make the whole summary unstable.

B Bill R d


19/19

By Bill Reed

Configuration

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