Hello Tuyen
In R1, we see the aggregate-address 172.16.0.0 255.255.0.0 command in the BGP configuration, and we want to send this aggregate address to BGP peers. This means that we must already have more specific routes in the routing table that we simply want to aggregate, such as 172.16.1.0/24 and 172.16.55.0/24 for example. These are not shown in the lesson, but would be there in a production environment.
Remember, a routing table lookup will look up the most specific routes. So even with the null static route, it would never be matched since something more specific would be matched, and successfully routed. If there are no routes that are more specific, then the packet would be dropped anyway so the null interface configuration wouldn’t make a difference.
I hope this has been helpful!
Laz