Re: If I announce 192.0.2.0/24, do I need a discard route? (Looking for a reference…)

Wed Jan 31 20:53:02 UTC 2024

> On Jan 31, 2024, at 12:30, Warren Kumari <warren at kumari.net> wrote:
> 
> Hey all,
> 
> This falls into the "Somebody is wrong on the Internet …" category.

Doesn’t everything eventually end up there?

> So, let's say I'm announcing some address space (e.g 192.0.2.0/24 <http://192.0.2.0/24>), but I'm only using part of it internally (e.g 192.0.2.0/25 <http://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.

Usually, but it’s not always necessary.

> 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…

Well… Unless you have some other more specific covering the rest of the space, yes, this is a risk.

> 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… 

I don’t know if it’s documented, but it’s certainly common sense.

> 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…. 

I think your “reason” paragraph above is quite sufficient explanation, no?

> Can anyone help me win this somewhat pointless argument?

No, because if you have an opponent who won’t buy the content of your reason paragraph above, you are arguing with someone who isn’t going to take any amount of fact or documentation over whatever ill conceived reality they have decided to live in.

You simply cannot win an argument when confronted with an opponent living in an alternative reality.

Owen

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