Add Book to My BookshelfPurchase This Book Online

Chapter 4 - Routing Protocols Used in TCP/IP

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

Routing Protocol Responsibilities
Routing protocols exist to keep routing tables accurate, even though the internetwork they are operating on will be changing due to equipment or line failures and the addition of new network segments.
To keep tables accurate, a routing protocol operation has two parts. The first sends advertisements (referred to as routing updates) out from a router, regarding the location of network numbers it knows about. The second receives and processes these routing updates in a way that keeps the router's routing table directing traffic efficiently.
There are two main types of routing protocols, the Interior Gateway Routing Protocol and the Exterior Gateway Routing Protocol. To understand the difference between these two, we need to define what an autonomous system is on an internetwork. Figure 4-1 shows a large internetwork split into three autonomous systems.
Figure 4-1: An internetwork split into three autonomous systems
The idea behind autonomous systems is as follows. Imagine there are three different internetworks, one managed by an American-based team, one by a UK-based team, and one by a team based in Japan. Each of these internetworks is managed in a different way and has different policies. Suppose these three internetworks need to be connected so they can exchange information, but the separate network management teams want to retain their own policies and want to control the routing updates received from the newly connected internetworks.
One way to meet these goals is to define an Exterior Gateway Routing Protocol process on the routers that connect the three internetworks. In Fig. 4-1, these are routers R1-1, R2-1 and R3-1. The Exterior Gateway Protocol limits the amount of routing information that is exchanged among these three internetworks and allows them to be managed differently.
Within each autonomous system, all routers would run an Interior Gateway Routing Protocol, which assumes that the whole internetwork is under the management of one body and, by default, exchanges routing information freely with all other routers in the same autonomous system. Sophisticated Interior Gateway Routing Protocols, such as Cisco's IGRP, however, have the functionality to act as Exterior Gateway Routing Protocols. As with most things in computer networking, clear-cut definitions are hard to come by.
Exterior Gateway Routing Protocols and multiple autonomous system numbers are used on the Internet but rarely in the commercial environment, so we shall concentrate mainly on the Interior Gateway Routing Protocols in this chapter.
The world of Interior Gateway Routing Protocols is split into two camps, one known as distance vector and the other as link state. In the next section, we discuss two distance vector routing protocol algorithms, RIP and IGRP, followed by a discussion of the hybrid EIGRP. After that, we shall look at two link state protocols, OSPF and Integrated IS-IS.

 


 
Books24x7.com, Inc © 2000 –  Feedback