Default Routes and RIP

Cisco's RIP implementation supports the propagation of a default route, also known as 0.0.0.0/0. When RIP finds a default route in its routing table, it automatically advertises this in the RIP update.

One important thing to remember here is that the default route must have a valid metric. For example, if the default route is learned through OSPF and the metric is 20, RIP will advertise this router with a metric of infinity (16). So, for this situation, the default-metric command must be used under the router rip command to ensure that the proper metric is assigned to the update.

Classless and classful IP routing concepts play an important role, especially with default routes. With classful IP routing, if the router receives a packet destined for a subnet that it does not recognize and the network default route is missing in the routing table, the router discards the packet. Figure 2-9 explains this behavior.

Figure 2-9. Classful IP Routing

Figure 2-9. Classful IP Routing

Here, Host X is sending traffic to the 131.108.3.0/24 subnet. Router R1 will discard these packets because it does not have a route for 131.108.3.0/24. Traffic will not be send to the default route because of the classful nature of routing.

If R1 enables IP classless routing, R1 will forward traffic to the default route.

Enabling IP classless routing is recommended when default network or default routes are used.

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