Content

| 3.2 |  | Default Routing |
| 3.2.1 |  | Default routing overview |

It is not feasible, or even desirable, for every router to maintain routes to every possible destination. Instead, routers keep a default route or a gateway of last resort. Default routes are used when the router cannot match a destination network with a specific entry in the routing table. The router must use the default route, or the gateway of last resort, to send the packet to another router. The next router will have a route to that destination or its own default route to a third router. If it is a default route to a third router, that router must have the route to the destination or another default route, and so on. Eventually, the packet should be routed to a router that has a route to the destination.

Using default routes keep routing tables small is a key scalability feature. They make it possible for routers to forward packets destined to any Internet host without having to maintain a table entry for every destination network. Default routes can be statically entered by an administrator or dynamically learned through a routing protocol.

Before routers can dynamically exchange default information, an administrator must configure at least one router with a default route. An administrator can use two different commands to statically configure default routes,

ip route 0.0.0.0 0.0.0.0 and ip default-network.

The following sections explore these two methods in detail.