EIGRP Chapter 6 Proprietary Routing

8/8/2019 EIGRP Chapter 6 Proprietary Routing

1/16

Chapter 6 EIGRP

Enhanced Interior Gateway

Routing Protocol (EIGRP) is aCisco-proprietary routing protocol

based on IGRP.


2/16

EIGRP Advantages

j Unlike IGRP, which is a classful routing

protocol, EIGRP is classless.

j EIGRP boasts faster convergence times,

improved scalability, and superior handling of

routing loops.

j EIGRP can replace Novell RIP and AppleTalk

Routing Table Maintenance Protocol (RTMP),serving both IPX and AppleTalk networks with

powerful efficiency.


3/16

EIGRP: Hybrid?

j EIGRP is an advanced distance-vectorrouting protocol that relies on featurescommonly associated with link-state

protocols.j OSPF's best traits, such as partial updates

and neighbor discovery, are similarly put touse by EIGRP.

j OSPF requires mastery of a complexterminology and command set. HoweverEIGRP's advanced features can be easilyimplemented and maintained.


4/16

EIGRP vs IGRP

j Cisco released EIGRP in 1994 as a scalable,improved version of its proprietary distance-vectorrouting protocol, IGRP.

j IGRP and EIGRP are compatible, although EIGRPoffers multiprotocol support and IGRP does not.

j EIGRP uses a metric that is 32 bits long, and IGRPuses a 24-bit metric. By multiplying or dividing by256, EIGRP can easily exchange information withIGRP.

j EIGRP also imposes a maximum hop limit of 224,slightly less than IGRP's generous 255.

j Redistribution is automatic between IGRP andEIGRP as long as both processes use the same

autonomous system (AS) number.


5/16

EIGRP Fundamentals

j EIGRP routers converge quickly because they rely ona state-of-the-art routing algorithm called theDiffusing Update Algorithm (DUAL)

j EIGRP makes efficient use of bandwidth by sendingpartial, bounded updates and its minimalconsumption of bandwidth when the network isstable.

j EIGRP offers full support for classless IP byexchanging subnet masks in routing updates.

j EIGRP supports IP, IPX, and AppleTalk throughprotocol-dependent modules (PDMs).

j PDMs protect EIGRP from painstaking revision.Evolution of a routed protocol, such as IP, may

require a new protocol module, but not necessarily areworking of EIGRP itself.


6/16

EIGRP support for IPX

j Time-driven protocols, IPX RIP and SAP,generate updates every 60 seconds bydefault. These updates can crowd low-speed

WAN links, especially in large internetworks.jAn EIGRP router will receive routing and

service updates and then update otherrouters only when changes in the SAP or

routing tables occur. Routing updates occuras they would in any EIGRP network - usingpartial updates, thus conserving bandwidthon the low-speed WAN links.


7/16

EIGRP Terminologyj Neighbor table - Each EIGRP router maintains a

neighbor table listing adjacent routers, comparable tothe OSPF adjacency database.

j Topology table - Every EIGRP router maintains atopology table for each configured network protocolshowing all learned routes to a destination.

j Routing table - EIGRP chooses the best routes to adestination from the topology table and places theseroutes in the routing table.

j Successor- A successor is a route selected as theprimary route to use to reach a destination. Multiplesuccessors for a destination can be retained in therouting table.

j Feasible successor- A feasible successor is a

backup route. Multiple feasible successors for adestination can be retained in the to olo table.


8/16

Neighbor Discovery and

Recoveryj EIGRP routers actively establish relationships with

their neighbors, much the same way that OSPFrouters do.

j EIGRP routers establish adjacencies with neighbor

routers by using small hello packets.

j An EIGRP router assumes that, as long as it isreceiving hello packets from known neighbors, thoseneighbors (and their routes) remain viable.

j

By forming adjacencies, EIGRP routers dynamicallylearn of new routes that join their network, identifyrouters that become either unreachable or inoperableas well as rediscover routers that had previouslybeen unreachable


9/16

Reliable Transport Protocol

j EIGRP is protocol-independent (i.e., it doesnot rely on TCP/IP to exchange routinginformation the way that RIP, IGRP, and

OSPF do).j To stay independent of IP, EIGRP uses its

own proprietary transport-layer protocol toguarantee delivery of routing information:

RTP.j EIGRP can call on RTP to provide reliable or

unreliable service as the situation warrants.


10/16

DUAL Finite State Machine

j The centerpiece of EIGRP is the Diffusing UpdateAlgorithm finite-state machine, EIGRP's route-calculationengine.

j An FSM is an abstract machine that defines a set of

possible states something can go through, what eventscause those states, and what events result from thosestates. Designers use FSMs to describe how a device,computer program, or routing algorithm will react to a setof input events.

j

The DUAL FSM contains all the logic used to calculateand compare routes in an EIGRP network.

j DUAL tracks all the routes advertised by neighbors anduses the composite metric of each route to compare them.DUAL also guarantees that each path is loop-free.Lowest-cost paths are then inserted by the DUAL protocol

into the routing table.


11/16

EIGRP Packet Types

The five EIGRP packet types are:

j Hello used to discover, verify, and rediscover neighborrouters. EIGRP routers send hellos at a fixed but configurableinterval, called the hello interval. The default hello interval

depends on the bandwidth of the interface, 60 seconds for1.54 mbps or less and 5 seconds for more than 1.54 mbps.Hellos are sent multicast to IP address 224.0.0.10.

j Acknowledgment sent as unicast to indicate receipt of anyEIGRP packet during a "reliable" exchange.

j Update - used when a router discovers a new neighbor or

when a router discovers a topology change. Sent unicastand reliably.

j Query - can be multicast or unicast, used when routers needspecific information from one or all neighbors. Sent reliably.

j Reply - used to respond to a query. Always sent as a

unicast.


12/16

EIGRP Neighbor Tables

j DUAL selects alternate routes based on the tableskept by EIGRP, and routers use the tables to track allthe routing information in an AS, not just the "best"routes.

j The most important table in EIGRP is the neighbortable and relationships tracked in the neighbor tableare the basis for all the EIGRP routing update andconvergence activity.

j The neighbor table contains information aboutadjacent neighboring EIGRP routers.

j A neighbor table is used to support reliable,sequenced delivery of packets.

j An EIGRP router can maintain multiple neighbor

tables, one for each PDM running (e.g., IP, IPX, andAppleTalk) routed protocols.


13/16

EIGRP Routing Table

jThe routing table contains the routes

installed by DUAL as the best loop-free

paths to a given destination

jEIGRP will maintain up to four routes

per destination. These routes can be of

equal or unequal cost.

jEIGRP routers maintain a separate

routing table for each routed protocol.


14/16

EIGRP Topology Table

j EIGRP uses its topology table to store all the information itneeds to calculate a set of distances and vectors to allreachable destinations.

j EIGRP maintains a separate topology table for each routedprotocol. The topology table is made up of all the EIGRP routingtables in the autonomous system.

j EIGRP sorts the topology table so that the successor routes areat the top, followed by feasible successors. At the bottom,EIGRP lists routes that DUAL believes to be loops in thetopology table.

j

Entries in the topology table can be in one of two states: activeor passive.

j A passive route is one that is stable and available for use. Anactive route is a route in the process of being recomputed byDUAL.


15/16

Configuring EIGRP

j To enable EIGRP and define the autonomoussystem:router(config)# router eigrp

autonomous-system-number

j Indicate which networks are part of the EIGRPautonomous system on the local router.router(config-router)# network

network-number

j When using serial links, it is important to use thebandwidth on the interface to change thebandwidth used for calculating routing metrics. Ifyou do not change the bandwidth for theseinterfaces EIGRP assumes the default bandwidthon the link instead of the true bandwidth.

router(config-if)# bandwidth


16/16

Configuring EIGRP

j Cisco also recommends that you add the followingcommand to all of your EIGRP configurations:

router(config-if)# eigrp log-

neighbor-changes

This command enables the logging of neighboradjacency changes to monitor the stability of therouting system and to help detect problems.

j EIGRP automatically summarizes routes. In mostcases, auto summarization is a good thing; it keepsrouting tables as compact as possible.

To turn off auto-summarization, use the followingcommand:

router(config-router)#no auto-

Documents

EIGRP Chapter 6 Proprietary Routing