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| 3.2 |  | Default Routing |
| 3.2.3 |  | Default routing with IGRP |

IGRP treats 0.0.0.0/0 routes differently. In fact, IGRP does not recognize the network 0.0.0.0/0 and will not include it in updates. To configure a dynamic exchange of default information in an IGRP network, the ip default-network command must be used. The ip default-network command can be used to flag a route to any IP network, not just 0.0.0.0/0, as a candidate default route. Use the following command syntax:

> Router(config)#ip default-network ip-network-address

An example of this command is shown in the following internetwork figure. An asterisk in the routing table denotes a candidate default route. The candidate default route will be considered, along with any other candidates for the role as gateway of last resort.

As boundary router, RTB must be manually configured to send default traffic to its link partner, the ISP router. A 0.0.0.0/0 route can be configured on RTB, but this route will not be propagated by IGRP to the other routers. To avoid manually configuring 0.0.0.0/0 routes on all routers, configure RTB to flag its route to 207.21.20.0/24 as a candidate default route, as shown in the following:

> RTB(config)#ip default-network 207.21.20.0

The network 207.21.20.0/24 now has special properties as an exterior network. The exterior network is the outside network that serves as a gateway of last resort. RTB will send this information in IGRP routing updates to RTA and RTC. These routers can now dynamically learn that network 207.21.20.0/24 is an exterior network, making RTB the gateway of last resort for both of these routers. Both RTA and RTC will propagate this route, flagged as a candidate default, to other IGRP neighbors, if present.

Unlike a static 0.0.0.0/0 route configuration, the

ip default-network command provides an administrator with a great deal of flexibility. In complex topologies, several networks can be flagged as candidate defaults. Routers can then choose from among the available candidates to pick the lowest cost route.

Running IGRP,

ip default-network command must be used to enable the exchange of default information. Using RIP a 0.0.0.0/0 route will usually suffice. However, use the ip default-network command on a RIP router, but RIP routers propagate IP default networks as 0.0.0.0/0 routes. In other words, a RIP router configured with the ip default-network 192.168.1.0 command will send neighbors a route to 0.0.0.0/0, not a flagged route to 192.168.1.0.

> Note: The

ip default-network command is classful, which means that if the router has a route to the subnet indicated by this command. It installs a static route to the major net and then flags that route as a candidate default route.

| Lab Activity e-Lab Activity: Default Routing with IGRP This lab is to make IGRP propagate a static default route.

| Web Links The IP Routing Protocols http://www.cisco.com/univercd/cc/td/doc/product/ software/ssr90/rpc_r/54043.htm