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.Thesetwo routing protocols even share the same distance-vector routing technology: EIGRP uses the samecomposite routing metric as IGRP.EIGRP also supports all the same distance vectors, and theirmathematical weights, as does IGRP.EIGRP also uses IGRP's Variance feature to provide unequal-costload balancing.Note There are strong similarities between many of IGRP and EIGRP's basic components.These includethe formula for calculating their respective composite metrics, as well as equal or unequal-cost loadbalancing using the Variance mechanism.If you would like more information on any of these topics,refer to Chapter 10, "Interior Gateway Routing Protocol."http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2911.htm (2 of 16) [02/02/2001 11.39.04] Enhanced IGRPThere is only one minor difference in the algorithm that calculates the composite metric: The IGRPmetric is 20 bits long, whereas the EIGRP metric is 32 bits long.This difference results in the EIGRPmetric being larger by a factor of 256 than a comparable IGRP metric for any given route.The largermetric allows a better and finer mathematical comparison of potential routes.This minor difference is easily and automatically compensated for by EIGRP.EIGRP automaticallyadjusts the composite metric of IGRP routes and adjusts its own metric on routes being redistributed toIGRP routers.IGRP's and EIGRP's metrics are directly comparable.Therefore, they can be usedinterchangeably after translation.EIGRP does, however, track the translated IGRP routes as externalroutes.Note IGRP doesn't have any concept of internal and external routes.Consequently, EIGRP routes thatare translated and redistributed into an IGRP network are treated as IGRP routes.Automatic redistribution between IGRP and EIGRP will only occur if the two protocols are configuredwith the same autonomous system (AS) number.If they have different AS numbers, they will assumethat they are part of different networks (that is, autonomous systems).EIGRP's automatic metric adjustment mechanism enables IGRP and EIGRP to be fully integrated in anetwork via a simple mathematical function.External IGRP routes that are automatically adjusted byEIGRP can be directly compared to internal EIGRP routes.Cisco routers will always select the routewith the best metric (adjusted or otherwise) rather than automatically select the route of any particularprotocol.Therefore, an EIGRP router might decide that the best route is actually an external IGRP routerather than an internal EIGRP route.Migration from IGRP to EIGRP can be done gradually without incurring any network downtime.EIGRPcan be introduced into strategic areas of the network such as the backbone.Its automatic metrictranslation mechanism would enable the network administrator to replace IGRP with EIGRP in thosestrategic areas.Support for IGRP is integral to EIGRP so the network's functionality isn't compromised.The network administrator can then selectively extend its use in the network, until the migration toEIGRP is complete.At this point, some of the more advanced IP architectures, such as VLSM andclassless addressing, can be implemented.EIGRP's ImprovementsCisco also instituted several changes in EIGRP that were designed to improve its operational efficiencyrelative to IGRP.These two protocols are interoperable thanks to their mutual distance vectors, but theyoperate in very different ways.EIGRP reacts to topological change differently, advertises routesdifferently, and even has a different approach to updating entries in routing tables.In many ways, EIGRPbehaves more like a link-state routing protocol than it does a traditional distance-vector protocol.Yet,EIGRP uses the distance vectors and composite metric of IGRP.Consequently, EIGRP is sometimesreferred to as a hybrid routing protocol (or an advanced distance-vector protocol).It combines the bestfeatures of link-state routing with the best features of distance-vector routing.Properly designed andimplemented, an EIGRP network is extremely stable and efficient and converges rapidly after anytopological change.http://wwwin.cisco.com/cpress/cc/td/cpress/fund/iprf/ip2911.htm (3 of 16) [02/02/2001 11.39.04] Enhanced IGRPSome of the specific advantages of EIGRP include the following:Minimal consumption of bandwidth when the network is stable---During normal, stable networkoperation, the only EIGRP packets exchanged between EIGRP nodes are hello packets.Thissimple handshake enables the EIGRP routers to know that all remains well in the network.Efficient use of bandwidth during convergence---EIGRP only propagates changes to the routingtable, not the entire routing table.Also, updates are advertised only after a topological changerather than on a strict, periodic basis.These updates are also only transmitted to those EIGRProuters that need to know of the change.This is known as a partial, bounded update.Rapid convergence---EIGRP routers store every path they have learned to every destination in thenetwork.Therefore, a router running EIGRP can quickly converge on an alternative route after anytopological change.Support for VLSM and CIDR---EIGRP supports the definition of network and host numbers on anybit boundary, per interface, for both IP addresses and subnet masks.Complete independence from routed protocols---EIGRP is designed to be completely independentof routed protocols.Support for routed protocols is via individual, protocol- specific modules.Therefore, evolution of a protocol, such as IP, won't threaten EIGRP with obsolescence.Nor willsuch technological advances force a painful revision of EIGRP.The features highlighted in this list are examined in more detail throughout this chapter.Combined, theymake a compelling case for migrating to EIGRP.New Features Found in EIGRPEIGRP enjoys many new technologies, each of which represents an improvement in operating efficiency,rapidity of convergence, or feature/functionality relative to IGRP and other routing protocols.Thesetechnologies fall into one of the following four categories:Neighbor discovery and recoveryReliable Transport ProtocolDUAL finite-state machineProtocol-specific modulesIn addition to these categories of technological improvements, EIGRP was also treated to some othersubstantial enhancements relative to IGRP
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