In this case, you can conditionally advertise a default-route by using the advertise-map, exist-map, non-exist map.
neighbor 11.11.67.7 advertise-map adv_default exist-map to_R3- advertise-map Specifies the BGP route that will be advertised if the conditions of the exist map or non-exist map are met.
- exist-map Specifies the conditional routes that need to be in local route table in order for BGP to advertise the routes defined in advertise-map.
- non-exist-map Specifies the conditional routes that must not be in local route table before BGP starts to advertise routes defined in advertise-map
Verify via:
R1#show ip route bgp
11.0.0.0/8 is variably subnetted, 5 subnets, 2 masks
B 11.11.6.0/24 [20/0] via 11.11.17.7, 00:09:30
B 11.11.7.0/24 [20/0] via 11.11.17.7, 00:09:31
B* 0.0.0.0/0 [20/0] via 11.11.17.7, 00:08:22