Cisco Systems 78-11741-02 manual Multiple Routing Protocols, Interior Gateway Protocols

IP Overview

IP Routing Protocols

Note Many routing protocol specifications refer to routers as gateways, so the word gateway often appears as part of routing protocol names. However, a router usually is defined as a Layer 3 internetworking device, whereas a protocol translation gateway usually is defined as a Layer 7 internetworking device. The reader should understand that regardless of whether a routing protocol name contains the word “gateway,” routing protocol activities occur at Layer 3 of the Open System Interconnection (OSI) reference model.

Interior Gateway Protocols

Interior gateway protocols are used for routing networks that are under a common network administration. All IP interior gateway protocols must be specified with a list of associated networks before routing activities can begin. A routing process “listens” to updates from other routers on these networks and broadcasts its own routing information on those same networks. Cisco IOS software supports the following interior routing protocols:

•On-Demand Routing (ODR)

•Routing Information Protocol (RIP)

•Interior Gateway Routing Protocol (IGRP)

•Open Shortest Path First (OSPF)

•Enhanced IGRP (EIGRP)

•Integrated IS-IS

Exterior Gateway Protocols

Exterior gateway protocols are used to exchange routing information between networks that do not share a common administration. IP Exterior Gateway Protocols require the following three sets of information before routing can begin:

•A list of neighbor (or peer) routers with which to exchange routing information

•A list of networks to advertise as directly reachable

•The autonomous system number of the local router

The exterior gateway protocol that is supported by Cisco IOS software is Border Gateway Protocol (BGP).

Multiprotocol BGP is an enhanced BGP that carries routing information for multiple network-layer protocols and IP multicast routes. BGP carries two sets of routes, one set for unicast routing and one set for multicast routing. The routes associated with multicast routing are used by Protocol Independent Multicast (PIM) to build data distribution trees.

Multiple Routing Protocols

You can configure multiple routing protocols in a single router to connect networks that use different routing protocols. You can, for example, run RIP on one subnetted network and IGRP on another subnetted network, and exchange routing information between them in a controlled fashion. The available routing protocols were not designed to interoperate, so each protocol collects different types of information and reacts to topology changes in its own way.

Cisco IOS IP Configuration Guide

IPC-3