network protocols handbook.pdf

273Protocols GuideCisco ProtocolsProtocol NameIGRP: Interior Gateway RoutingProtocolProtocol DescriptionThe Interior Gateway Routing Protocol (IGRP) is a routing protocolto provide routing within an autonomous system (AS). In themid-1980s, the most popular interior routing protocol was theRouting Information Protocol (RIP). Although RIP was quite usefulfor routing within small- to moderate-sized, relatively homogeneousinternetworks, its limits were being pushed by networkgrowth. The popularity of Cisco routers and the robustness ofIGRP encouraged many organizations with large internetworksto replace RIP with IGRP. Cisco developed Enhanced IGRP inthe early 1990s to improve the operating efficiency of IGRP.IGRP is a distance vector Interior Gateway Protocol (IGP). Distancevector routing protocols mathematically compare routesusing some measurement of distance. This measurement isknown as the distance vector. Distance vector routing protocolsare often contrasted with link-state routing protocols, which sendlocal connection information to all nodes in the internetwork.To provide additional flexibility, IGRP permits multipath routing.Dual equal-bandwidth lines can run a single stream of traffic inround-robin fashion, with automatic switchover to the secondline if one line goes down. Multiple paths can have unequal metricsyet still be valid multipath routes. For example, if one pathis three times better than another path (its metric is three timeslower), the better path will be used three times as often. Onlyroutes with metrics that are within a certain range or variance ofthe best route are used as multiple paths. Variance is anothervalue that can be established by the network administrator.Protocol Structureof entries in each of these three sections of updatemessages. The first entries (Ninterior) are taken tobe interior, the next entries (Nsystem) as being system,and the final entries (Nexterior) as exterior.• Checksum -- IP checksum which is computed usingthe same checksum algorithm as a UDP checksum.Related protocolsEIGRP, EGP, BGP, GRE, IP, TCP, RIPSponsor SourceIGRP is a Cisco protocol.Referencehttp://www.cisco.com/univercd/cc/td/doc/cisintwk/ito_doc/igrp.htmInterior Gateway routing Protocol8 16 24 32bitVersion Op code Edition ASystemNinterior Nsystem Nexterior Checksum• Version -- IGRP version number (currently 1).• Opcode -- Operation code indicating the messagetype: 1 Update; 2 Request.• Edition -- Serial number which is incremented wheneverthere is a change in the routing table.• Asystem -- Autonomous system number. A gatewaycan participate in more than one autonomous systemwhere each system runs its own IGRP. For eachautonomous system, there are completely separaterouting tables. This field allows the gateway to selectwhich set of routing tables to use.• Ninterior, Nsystem, Nexterior -- Indicate the number