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Chapter 4 - Routing Protocols Used in TCP/IP

Cisco TCP/IP Routing Professional Reference
Chris Lewis
  Copyright © 1999 The McGraw-Hill Companies, Inc.

Summary
In this chapter we examined how routers automatically update their routing tables to keep optimal routes to all network numbers, in spite of a changing internetwork topology. At the high level, there are two types of routing protocol, Interior and Exterior Gateway Protocols.
Interior Gateway Protocols are designed to exchange full routing information between all routers in the same autonomous system. An autonomous system is the largest entity in a routing hierarchy. Exterior Gateway Protocols are designed to transfer information between different autonomous systems in a controlled fashion.
Interior Gateway Protocols are further subdivided into distance vector and link state protocols.
Distance vector protocols only exchange information with neighbors on directly connected segments, using broadcast routing updates. It is the duty of each router to pass on route information learned from one neighboring router to another. These routing updates perform the function of transferring route information and act as keep-alives. Virtually all the routing table entries are sent in each routing update. Distance vector protocols require the use of several mechanisms to maintain an optimal routing environment, including Split Horizon, Poison Reverse, triggered updates, and hold-downs.
Link state protocols use separate mechanisms to update other routers and to send keep-alive messages. Routes are advertised using link state advertisements, which are explicitly sent to every other router in a given routing area. Keep-alives are sent in the form of small hello packets between routers. Sending LSAs to all routers in a given area enables each router to generate a complete map of that routing area. Link state protocols apply the Dijkstra algorithm to this database to generate routes for entry into the routing table.
In general, link state protocols scale better for very large internetworks, producing acceptable convergence times and less routing protocol traffic in the event of link failures. By "very large," I mean an internetwork servicing over 1500 locations. Link state protocols, however, have to deal with more internal structures to process and thus require more router CPU and memory.
In a Cisco environment, you have the choice of IGRP, EIGRP, or OSPF as the Interior Gateway Routing Protocol. In most cases, IGRP is adequate and is the most familiar to network administrators who have been accustomed to RIP. EIGRP and OSPF can provide better convergence times, but are more complex to customize. EIGRP, and particularly OSPF, require more RAM and a faster processor in the router device for adequate performance.
For most users, if an External Gateway Protocol is necessary at all, static routing is normally adequate. Finally we looked at the new IPv6 protocol. This protocol will be of most interest to ISPs and is unlikely to be widely used in private networks for many years.

 


 
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