Default routes can be injected into BGP via one of three ways:
- Using the network command
- using the redistribute command
- using the neighbor default-originate BGP sub-command
When using the network command, a route to 0.0.0.0/0 must already exist in the local routing table, and the network 0.0.0.0 command is required.
Using redistribution requires default information originate. The default route must already be in the routing table, then the redistribute command could be used to redistribute the route, Cisco IOS also requires default-originate BGP sub-command.
Using the neighbor default-originate BGP sub-command, does not add a route to the table, instead it causes the advertisement of a default to the specified neighbor. This method doesn’t even check to see if a default route exists in the local routing table. You can force it to check by using the route-map option check-default command.