BGP advertising a default route

Default routes are used by routing devices to forward packets to destination networks that are not specifically defined in entries within the routing table.

There are a couple of ways to configure a router running BGP to advertise a default route to its BGP peers:

• Configure a static route to the 0.0.0.0/0 network pointing to the null 0 interface by issuing the ip route 0.0.0.0 0.0.0.0 null0 command. Then use the network 0.0.0.0 command to inject the route into BGP.

• Configure a static route to the 0.0.0.0/0 network pointing to the null 0 interface by issuing the ip route 0.0.0.0 0.0.0.0 null 0 command. Then, redistribute this static route into BGP using the redistribute static command in conjunction with the neighbor X.X.X.X default-originate command to the neighbor to which you want to share that default route.

Links:

https://community.cisco.com/t5/networking-documents/how-to-configure-bgp-to-send-the-default-route-to-a-neighbor/ta-p/3131059

https://networklessons.com/bgp/how-to-advertise-networks-in-bgp/