Advertising a default route over BGP
JEREMY L. GADDIS , owner of the blog at http://evilrouters.net, has written an excellent tutorial on how to advertise a default route over BGP.
The most important part of the tutorial to actually advertise the default route is the following:
R3(config)# router bgp 33 R3(config-router)# neighbor 172.16.34.4 default-originate R3(config-router)# neighbor 172.16.35.5 default-originate
evilrouters.net : Advertising a default route over BGP