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Chapter 5

IP Routing

packet by default. If you are using default routing, you must use the
ip

classless command because no remote subnets will be in the rout-

ing table.

Since I have version 12.x of the IOS on my routers, the ip classless

command is on by default. If you are using default routing and this com-
mand is not in your configuration, you need to add it. The command is
shown here:

2501C(config)#ip classless

Notice that it is a global configuration mode command. The interesting

part of the ip classless command is that default routing sometimes
works without it, but sometimes it doesn't. Just to be on the safe side, you
should always turn on the ip classless command when you use default
routing.

Dynamic Routing

Dynamic routing is when protocols are used to find and update routing
tables on routers. (This is easier than static or default routing, but you use
it at the expense of router CPU processes and bandwidth on the network
links.) A routing protocol defines the set of rules used by a router when it
communicates between neighbor routers.

The two routing protocols discussed in this book are Routing Information

Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).

Enhanced Interior Gateway Routing Protocol (EIGRP) is an advanced

distance-vector routing protocol and is Cisco proprietary. Open Shortest
Path First (OSPF) is a non-proprietary link-state routing protocol used in the
TCP/IP stack. For information on other types of routing protocols such as
these, read CCNP: Routing Study Guide (Sybex, 2001), which covers the
CCNP: Routing exam from Cisco.

EIGRP utilizes the features of both distance-vector and link-state routing

protocols.

There are two types of routing protocols used in internetworks: interior

gateway protocols (IGPs) and exterior gateway protocols (EGPs).

IGPs are used to exchange routing information with routers in the same

autonomous system (AS). An AS is a collection of networks under a common

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