Hello sachin
BGP is very specific in what it advertises. it will only advertise what is in the routing table exactly as it is stated in the routing table including prefix.
If you want to use BGP to advertise a default route for outbound traffic, there are several ways to do this:
**network 0.0.0.0 command** - this will inject the default route into BGP only if this same default route is currently present in the routing table.
**redistribution from another routing protocol** will also inject the default route into BGP, again, only if it is currently in the routing table and if it has been learned from the specific routing protocol you are redistributing from.
using the default-information originate command which causes the default route to be artifically generated and injected into the BGP RIB even if it is not in the routing table. It will be advertised because it is in the RIB.
finally you can use the neighbor X.X.X.X defaut-originate command which is similar to the previous command except that the default route will be advertised only to this specific BGP neighbour and not to all BGP neighbours.
I hope this has been helpful!
Laz