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

Wed Jan 31 21:46:38 UTC 2024

On Wed, Jan 31, 2024 at 3:56 PM, William Herrin <bill at herrin.us> wrote:

> On Wed, Jan 31, 2024 at 12:30 PM Warren Kumari <warren at kumari.net> wrote:
>
> 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.
>
> Hi Warren,
>
> Your router won't announce 192.0.2.0/24 unless it knows a route to
> 192.0.2.0/24 or has been configured to aggregate any internal routes
> inside 192.0.2.0/24 to 192.0.2.0/24.
>

It that always true? I'd started off thinking that, but a friend of mine
(yes, the same one that started this  argument) convinced me that
some forms of BGP summarization/aggregation don't always generate a "local"
route…

I'd also thought that I'd seen this when redistributing an IGP into BGP,
and using that as a contributor to 'aggregate-address' on Cisco devices.
This is from a long time ago, and really hazy now, but I'd thought that any
contributor would cause that the aggregate-address route to be announced,
and a local hold down not to be created.  It's possible that a: I'm just
wrong b: this is not longer true, c: both of the above.

There are also some more inventive ways of getting routes into BGP, like
using ExaBGP as an example.

W

192.0.2.0/25 doesn't count; it needs to know a route to 192.0.2.0/24.
> Sending 192.0.2.0/24 to discard guarantees that the router has a route to
> 192.0.2.0/24.
>
> Historically, folks would put 192.0.2.0/24 on the ethernet port. Then,
> when carrier was lost on the ethernet port for a moment, the router would
> no longer have a route to 192.0.2.0/24, so it'd withdraw the announcement
> for 192.0.2.0/24. This is a bad idea for obvious reasons, so best
> practice was to put a low priority route to discard as a fall-back if the
> ethernet port briefly lost carrier.
>
> Regards,
> Bill Herrin
>
> --
> William Herrin
> bill at herrin.us
> https://bill.herrin.us/
>
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