Configure a default route on a Cisco router

The default route is the IP address of the next hop when no other routes are known.

To configure the default route to be 192.168.1.1:

config t
ip route 0.0.0.0 0.0.0.0 192.168.1.1

An interface can be used as an alternative to and IP address. To use serial0/0 for destinations not in the routing table, use:

ip route 0.0.0.0 0.0.0.0 serial 0/0

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courtesy http://www.tech-recipes.com/rx/79/configure-a-default-route-on-a-cisco-router/