3.2 Default Routing
3.2.2 Configuring static default routes
Creating an ip route to 0.0.0.0/0 is the simplest way to configure a default route. This is done using the following syntax:

Router(config)# ip route 0.0.0.0 0.0.0.0 [next-hop-ip-address | exit-interface]

To the Cisco IOS, network 0.0.0.0 /0 has special meaning as the gateway of last resort. All destination addresses match this route because a mask of all 0s requires none of the 32 bits in an address to be an exact match.

A route to 0.0.0.0 /0 is often called a "quad-zero route." Manually configuring 0.0.0.0/0 routes on every router might suffice in a simple network. You may want routers to dynamically exchange default routes in more complex situations. The exchange of default information works differently depending on the routing protocol being used and can create severe problems when improperly configured. Remember, default routes typically point to the outside world, so when they fail, everyone tends to notice!

Static routes to 0.0.0.0/0 are automatically propagated to other routers in RIP updates. The only way to stop this automatic update is to use a route filter, a configuration option that is discussed in Chapter 7, Route Optimization.

However, in IOS release 12.1, RIP does not propagate a static default route automatically. If you are using RIP and IOS 12.1, you must manually configure the RIP process to advertise the static default by issuing the network 0.0.0.0 command.

Alternately, you can use either the default-information originate command or the redistribute static command to configure static default route propagation. OSPF (regardless of the IOS version) requires the default-information originate command if you want to propagate static default routes. The following example illustrates this configuration for RIP (see the figure).

RTY(config)# ip route 0.0.0.0 0.0.0.0 172.16.1.2
RTY(config)# router rip
RTY(config-router)# default-information originate

Using the default-information originate command, an administrator can statically configure a single RIP router with a 0.0.0.0/0 route, and that default route will be propagated to other routers. The default-information originate command can also be used with OSPF to achieve the same effect.