You've understood correctly. You need to create a null/blackhole route within your routing table (static routes work best as it guarantees the route exists) in order to announce the /24 supernet if you're using longer subnets (/25 to /32). The route needs to exist in the routing table in order for it to be advertised. In all the prefixes I've configured and further broken up, I've always configured blackhole routes for the /24 with a distance of 254, never worked for me without it. Same deal for IPv6.

Hey all,

This falls into the "Somebody is wrong on the Internet …" category.

So, let's say I'm announcing some address space (e.g 192.0.2.0/24), but I'm only using part of it internally (e.g 192.0.2.0/25). I've always understood that it's best practice[0] to have a discard route (eg static to null0/discard or similar[1]) for what I'm announcing.

There are a bunch of reasons for this, but the standard (or easiest to explain one!) is what happens if this comes from some provider space, and they announce a supernet/covering route. If I *don't* have a discard/hold-down route, and a packet is sent to part of the space I'm not using (e.g 192.0.2.200), I would send it to the covering route, they would just send it back to the more specific, I'd return it to them, etc…

Many, but not all mechanisms that people use for advertising a route in BGP automagically create this sort of discard route (e.g Juniper's 'aggregate'), but I wasn't really able to find any useful documentation suggesting that if you announce a route, you should make sure that you have some route covering all of the space…

Perhaps there isn't really anything saying this (because it's obvious), but I'd really like to find something so that I can point at it….

Can anyone help me win this somewhat pointless argument?

[0]: Best practice as in "you should do this, unless you've got some weird corner case and have thought about it for more than a few seconds...."

[1]: Yes, in some cases I'll have e.g an interface that match the announcement, and that accomplishes the same thing.

[3]: E.g. 192.0.2.0/24 comes from a provider, and they are announcing something shorter.