Propagation of Default Routes

Usually, it is not enough to configure a default route on one router and be done with it. The default route must be shared with other routers, as depicted in Figure 3-10.

Figure 3-10 Propagation of a Default Route

In Figure 3-10, Router E is the exit point for traffic leaving the corporate network and heading out to the Internet. Router E is configured with a default route so that it doesn't have to hold the entire Internet routing table. This satisfies Router E's requirement to reach points on the Internet, but what about the other routers in the corporate network? They must also learn about this default route or they will not know how to forward a packet destined to the Internet (they will drop the packet). To inform others of the default route, Router E propagates the default route with the interior routing protocol. In the same way that it carries normal routes, the routing protocol carries the default route to all parts of the corporate network. As each router receives the default route, it can then forward traffic destined for the Internet. This process is called originating (or advertising) a default route. Because Router E starts it all, E is the originator.

Corporate Network

Router E

The Internet

Default

RIP, IGRP, OSPF, or EIGRP

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