7.4 The 0.0.0.0 Default Route
7.4.4 What to do instead of using 0.0.0.0 with IGRP
This section illustrates a previous point about IGRP behavior when a route to 0.0.0.0 is available. As previously mentioned, the 0.0.0.0 route is automatically installed as the local gateway of last resort for the router by the Cisco IOS software. No ip default- network 0.0.0.0 command is required. Also, as previously mentioned, IGRP does not advertise 0.0.0.0 to other routers, even if redistribute static and a default metric are configured. The main figure shows the network topology used in this scenario.

In the routing table from router A, you can see that the static route to null0 for 0.0.0.0 is entered as the gateway of last resort. (see RouterA#show ip route (1) command output).

The partial configuration from router A shows how this is done. (see RouterA#show running-config (1) command output). 

In the output of the debug ip igrp transactions command from router A, you can see that router A is not advertising 0.0.0.0. (see RouterA#debug ip igrp transactions (1) command output).

Remember that RIP advertised 0.0.0.0 automatically. The partial configuration from router A shows that a default metric has been configured, as well as redistribute static. Router A still does not advertise 0.0.0.0. (see RouterA#show running-config (2) command output).

In the output of the debug ip igrp transactions command from router A, you can see that router A is still not advertising 0.0.0.0. (see RouterA#debug ip igrp transactions (2) command output).

The partial configuration from router A shows how to get router A to use 0.0.0.0 as the local default route. It also shows how to advertise a different candidate default route --- 10.0.0.0 --- that other routers can consider as a possible gateway of last resort. The ip default network command is used to tag a route as a candidate default. (see RouterA#show running-config (3) command output).

A router can have multiple default networks configured; however, 0.0.0.0 is the preferred route when it is in the router routing table with an equal or better metric than any other candidate default route. Because both routes in router A are static, they have identical metrics.

The routing table from router A shows both static routes. Note that the asterisk (*) indicates a candidate default route. (see RouterA#show ip route (2) command output).

The routing table from router C shows that router C has received the router A advertisement for 10.0.0.0 and  installed it as a gateway of last resort. (see RouterC#show ip route command output).

The 0.0.0.0 route is not as useful for IGRP as it is for RIP. However, it is safer to use with IGRP for the same reasons that it is not as useful.

The simplest way to achieve the same effect for IGRP is to create and redistribute a static route for a fictitious network and flag it as the IP default network. You don't want to use a real network because you may not be able to control where the route for the real network is learned. It may not always be learned via the path on which you want your traffic to travel.

If you have multiple routers with exit points to other networks that you want to use as redundant links to the outside world, you can configure the same redistributed static route in each of them. If you want one to always be preferred over the others, give it a much better metric. Make certain that, for any router in the network, the metric for the preferred route is better than the metric it receives from the candidate default of another router.