Fundamentals of Default Routes

When a router is connected to the Internet, a default route is immensely useful. Without a default, the router will have to have a route entry for every destination address reachable over the Internet. As of this writing, such a routing table consists of more than 55,000 entries. With a default route, the router need only know about the destinations internal to its own administrative system. The default route will forward packets destined for any other address to the Internet service provider. In dealing with large routing tables, topology changes are an even bigger concern than the demands on memory. In a large internetwork, topology changes will occur more frequently, resulting in increased system activity to advertise and process those changes. Using a default route effectively "hides" the changes of more-specific routes, making the internetwork into which the default is advertised more stable.

Default routes are also useful on a smaller scale, within single autonomous systems. The same benefits of decreased memory and processor utilization can be gained in smaller internetworks, although the benefits decrease as the number of routes decreases.

Default routes are also very useful in hub-and-spoke topologies, such as the one in Figure 12.1. Here the hub router has a static route to every remote subnet. Entering new static routes in the hub router when a new subnet is brought online is a fairly trivial administrative task, but adding the routes to every spoke router might be much more time-consuming . By using default routes at the spoke routers, only the hub needs entries for every subnet. When a spoke router receives a packet for an unknown destination, it will forward the packet to the hub, which can in turn forward the packet to the correct destination.

Figure 12.1. Default routes greatly simplify the administration of static routing in a hub-and-spoke internetwork.

The spoke routers in Figure 12.1 are more correctly called "stub" routers. A stub router has only a single connection to another router. The routing decisions become very simple in such a device: The destination is either one of the router's directly connected networks ( stub networks ), or it is reachable via its single neighbor. And if the single neighbor is the only next -hop routing choice, the stub router has little need for a detailed routing table. A default route may be sufficient.

As with other summary routes, the trade-off with default routes is a loss of routing detail. The stub routers in Figure 12.1, for instance, have no way of knowing whether a destination is unreachable. All packets to unknown destinations are forwarded to the hub router and only then is reachability determined. Packets to nonexistent addresses should be infrequent in an internetwork. If for some reason they are not, a better design choice might be to allow the stub routers to have a full routing table so that unknown destinations can be determined as soon as possible.

Another problem with loss of routing detail is shown in Figure 12.2. These routers form a nationwide corporate backbone, and large local internetworks are connected to each of the backbone routers. The Los Angeles backbone router is receiving default routes from both San Francisco and San Diego. If Los Angeles must forward a packet to Seattle and has only the two default routes, it has no way of knowing that the best route is via San Francisco. Los Angeles may forward the packet to San Diego, in which case the packet will use a small portion of some very expensive bandwidth before it belatedly reaches its destination. Using default routes on this backbone is a bad design decision ^[2] but illustrates how hiding route details with a default route can lead to suboptimal routing.

^[2] Having each backbone router advertise only a default route into its local internetwork, on the other hand, can be a very good design choice.

Figure 12.2. If the Los Angeles router knows only the default routes advertised by San Francisco and San Diego and has no more specific details about the topology behind those two routers, it cannot route efficiently .