EIGRP Explanation & Configuration By Bill Reed

We Will Examine the features of EIGRP Discuss why EIGRP is known as a Hybrid Routing Protocol Identify the way EIGRP Routers establish relationships with neighbours using the DUAL algorithm, and then communicate with them. Describe the configuration of EIGRP.

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

By Bill Reed 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 d istance Vector (DV) EIGRP does not send its entire Routing Table in its 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 basic Terminology How EIGRP works

By Bill Reed And some packet types How EIGRP works And finally the tables

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 fsm lives here) A word about DUAL How EIGRP works

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

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

By Bill Reed How EIGRP works Neighbour Discovery TOM Neighbour Table Topology Table Routing Table Hello packet sent MULTICAST UNRELIABLY Hello packet received AS & K values examined IF AS & K values match Neighbour added

By Bill Reed How EIGRP works Neighbour 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 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 a Cisco proprietary protocol. Updates can contain multiple routes And can be advertising internal or external routes

By Bill Reed How EIGRP works Route 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 hop Delay Bandwidth MTU Hop count Reliability Load Prefix length Destination 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 weight K3 = Delay weight K4 = Secondary reliability weight K5 = Primary reliability weight Be = Bandwidth component * Dc = Delay component *

By Bill Reed How EIGRP works Route 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 router Origination AS number External protocol metric External 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 works Route Discovery TOM Neighbour Table Topology Table Routing Table DICK MARY Once updates have been exchanged The Topology table can be compiled To reflect the network. Network a# Network b# MARY

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 router must 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 works Route 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 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.

By Bill ReedConfiguration