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.