BGP Advertising a route using the Null0 exit interface

In order to advertise a network in BGP, you must have that network in the routing table. If it is not in the routing table, then you can add it with a Null exit interface so that it will be advertised. For example, the following configuration will advertise the 172.16.0.0/16 network using BGP.

R1(config)#ip route 172.16.0.0 255.255.0.0 null 0
R1(config)#router bgp 1
R1(config-router)#network 172.16.0.0 mask 255.255.0.0

This is also useful when applying BGP summarization. The summary route is not in the routing table, with the exact subnet mask, so BGP will not advertise it. However, you can add the summary route as a static route with a Null0 exit interface in the same manner.

Links:

https://networklessons.com/bgp/troubleshooting-bgp-route-advertisement/#BGP_Summarization

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