Default Route and IGRP

In Cisco routers, IGRP does not recognize the 0.0.0.0/0 route as the default route. It uses its own method of propagating default route with the ip default-network command.

The ip default-network command specifies a major network address and flags it as a default network. This major network could be directly connected, defined by a static route, or discovered by a dynamic routing protocol. The network specified by the ip default-network command must be in the routing table before the command takes effect. If the route specified is not in the routing table, no default route will be propagated. Figure 4-4 demonstrates how the ip default-network command works.

Figure 4-4. Propagating a Default Route for IGRP

In Figure 4-4, Router 1 is connected to the remote site through a DS-3 link. Router 1 now wants to send a default route to Router 2 and to all the routers in the remote site network. In IGRP, the route to 0.0.0.0 is not recognized as a default route; instead, Router 1 must configure ip default-network 192.168.1.0 to flag the route 192.168.1.0 as the default route. Router 1 will send a routing update of 192.168.1.0 and will flag it as a default route.

When the routers in the remote site network receive the update for 192.168.1.0, they will mark it as default route and will install the route to 192.168.1.0 as the gateway of last resort.