Whenever one uses aggregates, it is helpful to use null0 routes. Not just for BGP.
Suppose you have multiple sites, the addresses of which you are summarizing, and one of these sites go down. You are still advertising the whole aggregate. To prevent traffic for that downed site from going back and forth in your network, discard it right there.
The previous posting is quite correct. Just wanted to add that since the Internet is quite a huge monster, when there is a route goes down, you would not want BGP routers to update every other routers about a route down. You would rather still advertising a lost route (part of the summarised route pointing to null 0) to other while calmly (need to concentrate here) and quitely fix the problem (and not let everybody know about it).
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...