Using Default-Originate Command

 

From Command Lookup Tool:

neighbor default-originate

To allow  a BGP speaker (the local router) to send the default route 0.0.0.0 to a neighbor for use as a default route, use the neighbor default-originate command in address family or router configuration mode. 

LAB Setup:
Router A

interface Serial1/0
 ip address 10.1.1.1 255.255.255.252
 serial restart-delay 0

router bgp 1
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.1.1.2 remote-as 1
 no auto-summary

Router B


interface Serial1/0
 ip address 10.1.1.2 255.255.255.252
 serial restart-delay 0

router bgp 1
 no synchronization
 bgp log-neighbor-changes
 neighbor 10.1.1.1 remote-as 1
 neighbor 10.1.1.1 default-originate
 no auto-summary
!
ip route 0.0.0.0 0.0.0.0 10.1.33.251

Here in router A you will see that router B's default route is successfully advertised to Router A by issuing Show ip bgp command and Show ip route.