9.3
Issues of Redundancy, Symmetry and Load Balancing
9.3.3
How to use dynamically learned default routes
Defaults is a powerful technique in minimizing the amount of routes a router has to learn and for providing networks with redundancy in the event of failures and connectivity interruptions. It is important to understand how default routing works, because although it makes life easier when configured correctly, poorly designed default routing can make life more difficult.

By definition, a default route is a route in the IP forwarding table that is used if a routing entry for a destination does not exist. In other words, a default route is a last resort in case specific route information for a destination is unknown.

The universally known default route is usually represented by the network/mask combination 0.0.0.0/0.0.0.0 (or 0/0); any system advertising this route will be representing itself as a gateway of last resort for other systems. The Figure illustrates such an advertisement.

Most routing protocols, including BGP and almost all IGPs, can distribute a default route. To provide redundancy, you could be receiving defaults from multiple sources. In the context of BGP, the local preference can be set for the default to give a degree of preference over which default is primary and which is backup. If one default goes away, the other will take its place.

In the example on the left in the Figure, AS1 is connected to AS2 via two connections from a single router. In this example, AS1 is learning 0/0 from two links and giving preference by setting the local preference to 100 on the primary link through 1.1.1.1, and 50 (or any number smaller than 100) on the backup link. This would set the gateway of last resort to 1.1.1.1.

In the multiple routers scenario (example on the right in the Figure), the same behavior can be achieved as long as IBGP is running inside the AS. Local preference, which is exchanged between routers, will determine the primary and backup links.