Leveraging Default Routing

Default routing scales the network, conserves resources, and simplifies routing information. A default route is a special route that tells the router how to reach unknown destinations—that is, destinations that arc absent from the routing table because they are neither learned through a routing protocol nor manually configured with static routes. A default route is a catchall: When the router doesn't know how to forward a packet (because the packet is destined for an unknown network), the router sends it to the next hop defined by the default route. Without the default route, the packet is dropped because the router has no idea how to forward it.

It might sound as though default routes exist to cover up routing mistakes, but that is not the case. With default routing, you purposely withhold routing information from the router so that its routing task is reduced and simplified. This reduces the size of the routing table and conserves router resources (memory and CPU). To make up for the missing routes (the missing information), you provide the router with a simple default route. It's like taking a burden off the router and simplifying its view of the world to only what it needs to know. Default routes can substantially reduce the overhead of routing protocols, and they are a necessity when you connect to the public Internet. Figure 3-9 describes the typical uses for default routing.

Figure 3-9 Default Routing and the Network of Widget, Inc.

Figure 3-9 Default Routing and the Network of Widget, Inc.

Widget Inc. Big Corporate Network

Router C

Widget Inc. Big Corporate Network

,fK J Router D

Router C

Default route = RTC

Branch Office

In Figure 3-9. Router D at Widget, Inc., is connected to the Internet through an ISP router, ISP-RT. Because Widget has only one connection to the Internet, Router D doesn't need to hold and manage a full Internet routing table—that's a lot of routes. Router D has only one way to get to the Internet: through ISP-RT. Router D already knows how to reach everything inside of Widget through the interior routing protocol, so traffic going anywhere else must be destined for the Internet. Widget configures a default route on Router D that points to ISP-RT (the FOS configuration of default routes follows in later sections). When Router D receives a packet destined for a non-Widget network, it uses the default route and sends it to ISP-RT.

NOTE Even if Router D maintains a full Internet table because it's directly connected to the ISP, it's not practical for all routers in the big corporate network to also maintain all Internet routes. Eventually, routers in the corporate network downstream of Router D will require default routing to reach the Internet.

Figure 3-9 includes another default routing situation. The branch office router, Branch-RT, has a locally attached LAN and one path out to the big corporate network. Branch-RT doesn't need a full-size routing table with thousands of routes from the big corporate network because two choices suffice: Either packets go to the branch office FAN or they go somewhere else in the world via Router C. Therefore, Widget, Inc., configures a default route on Branch-RT that points to Router C as the next hop. Subsequently, Router C does not need to send Branch-RT any routing updates Router C can have a passive interface or route filter on its link to Branch-RT, reducing overhead on the link between Branch-RT and Router C.

NOTE Router C still needs to know how to reach the branch office LAN, so Branch-RT needs to send a single route upstream to Router C.

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