CISCO IOS in a Nutshell

Chapter 8. IP Routing Topics

Routing Protocol Topics

To do the job right, a router needs information about how to reach various parts of the network. It can get this information through static routing (i.e., including commands in the configuration that specify explicitly how to reach certain networks) or by exchanging routing information with other routers. To share this information, the router uses a routing protocol such as RIP, IGRP, EIGRP, OSPF, IS-IS, or BGP. In this chapter, we discuss a number of topics that are common to all (or most) of these protocols.

Autonomous System (AS) Numbers

An autonomous system is a collection of routers that is under the control of one organization (for example, one corporation’s network). Within this one organization, the routers share routing information only among themselves. At a macroscopic level, autonomous systems can be viewed as single entities. An exterior routing protocol, such as BGP, only needs to worry about autonomous systems, and can ignore any structure inside of the autonomous system.

Properly speaking, AS numbers are assigned by ARIN (the American Registry for Internet Numbers) and are used only by BGP. A limited number are available, so they are assigned only to organizations that really need them.^[4]

However, IGRP and EIGRP use a unique number to define boundaries between groups of routers that share information. This number can be considered a “locally significant AS” or “local-AS,” which means the number is important only ...