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

[Tom Beecher <beecher () beecher cc>]

> 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.

By spec, a route cannot be put into Adj-RIB-Out and announced to a peer
UNLESS that route exists in Loc-RIB, with a resolvable next-hop. ( RFC
4721, 9.1.3 . Your friend may need this :) )

It's certainly possible that a BGP implementation exists that violates this
rule, or hides the fact that it's doing this, but if it's standards
compliant this is what should be happening.

On Wed, Jan 31, 2024 at 4:47 PM Warren Kumari <warren () kumari net> wrote:

> On Wed, Jan 31, 2024 at 3:56 PM, William Herrin <bill () herrin us> wrote:
>
> > On Wed, Jan 31, 2024 at 12:30 PM Warren Kumari <warren () 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 () herrin us
> > https://bill.herrin.us/