9.3
Issues of Redundancy, Symmetry and Load Balancing
9.3.4
Configuration example: dynamically learned defaults
It is important to control defaults in BGP because if they are originated randomly they could cause everybody serious problems; for example, if a BGP speaker that intends to originate default to a specific peer ends up flooding the default to all of its neighbors could pull in all the traffic from these surrounding ASs. Cisco provides a way to target the default toward a specific neighbor.

Note: Click on topology to view command outputs.

In the Figure, RTA is originating a default route 0.0.0.0 0.0.0.0 toward RTC only; other IBGP neighbors, such as RTF, will not get the default. RTA's configuration follows. The default-originate option of the neighbor command will cause 0/0 to be sent toward RTC. This is shown in the BGP and IP routing tables of RTC. The routing table of RTC indicates that RTC has dynamically learned the 0/0 default from RTA and has set its gateway of last resort to 172.16.20.2, which is RTA.

Defaults can also be originated over all BGP peers using the network 0.0.0.0 router command as long as the router has its own default. The following configuration can be used, assuming that RTA has a default route itself (default could be created via a static route).