Lab 21: Default Routing, Filtering, and Unequal-Cost Load Sharing in IGRP Networks-Part II

RIP Default Routing

A default route is necessary whenever connecting the Internet. Without it, the router would need a path to every single network in its routing table. A default route is configured to point to a gateway of last resort. When a router cannot find a specific match in its route table for a packet, it forwards that packet to the gateway of last resort. Cisco routers always perform classful route lookups, which means that they will not forward packets to a gateway of last resort unless the global ip classless command is set. The ip classless command is enabled by default in Cisco IOS Software Release 11.3 and later.

The concept of default routing varies by each routing protocol. Each routing protocol uses a specific method when defining and advertising a default route.

There are two steps to perform when configuring default routing with RIP:

Step 1. Define or flag a default network. There are two ways to accomplish this. First, RIP will recognize the address of 0.0.0.0 as a default route. Default routes like this are created by adding a static route of all 0s, as in ip route 0.0.0.0 0.0.0.0 a.b.c.d, to the router. When this static route is created, it is not necessary to redistribute it into RIP. It will be propagated automatically by RIP. The second way to flag a route as a default is through the ip default-network a.b.c.d command.

Step 2. Ensure that ip classless is enabled on the router. Without ip classless, routers will not forward traffic to the gateway of last resort.

In Figure 9-6, the dr_xavier router was added to the network from the previous examples. This router has a default route to the Internet, through Internet_router/Firewall. This example uses a default static route to mark the route as the default. Adding the ip route 0.0.0.0 0.0.0.0 206.191.240.2 command to the dr_xavier router propagates a default route to rogue and cyclops. The route table for the dr_xavier router marks this route with an *, meaning that the route is the candidate default. When the route is propagated to a downstream router, it becomes a gateway of last resort, as shown in Figure 9-6.

Figure 9-6. RIP Default Routing

Example 9-11 lists the configuration of the dr_xavier router.

Example 9-11 Relevant Portions of the dr_xavier Configuration

  !   router rip   network 128.200.0.0   network 206.191.240.0   !   ip classless   ip route 0.0.0.0 0.0.0.0 206.191.240.2   !