EIGRPEIGRP

Explanation & Explanation & ConfigurationConfiguration

By Bill Reed

By Bill Reed

We WillWe WillExamine the features of EIGRPExamine the features of EIGRP

Discuss why EIGRP is known as a Hybrid Discuss why EIGRP is known as a Hybrid Routing Protocol Routing Protocol

Identify the way EIGRP Routers establish Identify the way EIGRP Routers establish relationships with neighbours using the relationships with neighbours using the DUAL algorithm, and then communicate DUAL algorithm, and then communicate

with them.with them.

Describe the configuration of EIGRP.Describe the configuration of EIGRP.

By Bill Reed EIGRP FeaturesEIGRP 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

By Bill Reed EIGRP why Hybrid?EIGRP why Hybrid?

EIGRP is called a Hybrid Routing Protocol because it features some of the characteristics of both Distance Vector (DV) and Link State (LS) Routing Protocols.

EIGRP Uses Metric Calculation like distance Vector (DV)

EIGRP does not send it’s entire Routing Table in it’s updates, it sends only the changes like Link State

Protocols (LS)

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

By Bill Reed First some First some basic basic

TerminologyTerminology

How EIGRP worksHow EIGRP works

By Bill Reed And some And some packet packet typestypes

How EIGRP worksHow EIGRP works

And finally And finally the tablesthe tables

By Bill Reed

EIGRP operation is primarily based upon theEIGRP operation is primarily based upon theDUAL DUAL finite state machinefinite state machine

EIGRP operates by carrying out:EIGRP operates by carrying out:

Neighbour DiscoveryNeighbour Discovery

Route DiscoveryRoute Discovery

Route SelectionRoute Selection

Route Maintenance (DUAL Route Maintenance (DUAL fsmfsm lives here) lives here)

A word A word about about DUALDUAL

How EIGRP worksHow EIGRP works

By Bill Reed How EIGRP How EIGRP worksworks Neighbour DiscoveryNeighbour Discovery

Hello Packet

TOM

DICK

MARY

Hello = 60 Seconds for slow links

Hello = 5 Seconds for all other links

By Bill Reed How EIGRP How EIGRP worksworks Neighbour DiscoveryNeighbour Discovery

Hello Packet

TOM

DICK

MARY

Hello = 60 Seconds for slow links

Hello = 5 Seconds for all other links

By Bill Reed How EIGRP How EIGRP worksworks Neighbour DiscoveryNeighbour Discovery

TOM

Neighbour Table

Topology Table

Routing Table

Hello packet sent

MULTICAST

UNRELIABLY

224.0.0.10

Hello packet received

AS & K values examined

IF AS & K values match

Neighbour added

By Bill Reed How EIGRP How EIGRP worksworks Neighbour DiscoveryNeighbour Discovery

TOM

Neighbour Table

Topology Table

Routing Table

MARY

Once both neighbours have identified one another and matched both AS and K values.

An Adjacency has been formed

MARY

By Bill Reed How EIGRP How EIGRP worksworks Route DiscoveryRoute 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 a Cisco proprietary protocol.

Updates can contain multiple routesAnd can be advertising internal or external routes

By Bill Reed How EIGRP How EIGRP worksworks Route DiscoveryRoute Discovery

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 countReliabilityLoadPrefix 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 WeightK2 = Load weightK3 = Delay weightK4 = Secondary reliability weightK5 = Primary reliability weightBe = Bandwidth component *Dc = Delay component *

By Bill Reed How EIGRP How EIGRP worksworks Route DiscoveryRoute Discovery

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.

By Bill Reed How EIGRP How EIGRP worksworks Route DiscoveryRoute Discovery

TOM

Neighbour Table

Topology Table

Routing Table

DICKMARY

Once updates have been exchangedThe Topology table can be compiledTo reflect the network.

Network a#

Network b#

MARY

By Bill Reed How EIGRP How EIGRP worksworks Route SelectionRoute 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#

By Bill Reed How EIGRP How EIGRP worksworks Route MaintenanceRoute Maintenance

ROUTE MAINTENANCE (The DUAL Finite State Machine)

DUAL is the Algorythm that drives the routing 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.

By Bill Reed How EIGRP How EIGRP worksworks

Route Route SummarisationSummarisation

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.

By Bill Reed

ConfigurationConfiguration