On Wed, Jan 31, 2024 at 5:56 PM, Owen DeLong <owen@delong.com> wrote:
On Jan 31, 2024, at 13:46, 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…
It’s not ALWAYS true. For example, aggregate-address commands and equivalent on many platforms will cause the route to be announced if any component prefixes are present in the RIB on some platforms.
There are also some cases where “auto-summary” (does anyone actually use this? EVER?) will have a similar effect.
There are also some implementations where disabling synchronization or in many cases where synchronization has simply been deprecated by the implementor where any bap “network” statement (or equivalent) will result in announcement regardless of what’s in the RIB.
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.
Redistributing an IGP into BGP is almost always a bad idea. However, that aside, yes, as mentioned above, exactly what you are saying here is true with or without the redistribution on most platforms IIRC.
a: you are not wrong b: it’s still true on many platforms at least (though may be implementation dependent) c: no, but it’s certainly not best practice to behave in this way or depend on either of these behaviors for the other original reason you stated among other reasons.
With BGP, you really want to have a deterministic clean definition of what your router will do. As a general rule, if your peer is reachable, you usually want to advertise your originated routes to them and make damn sure your router can reach those some how no matter what.
Yah, agreed…. This seems "obvious", but is there actually anything that states this? I'm not really sure where I think that I'd find something authoritative *to* state something. Even though we claim to be network **engineers**, there isn't really very much documentation of correct behavior — there are things like the MANRS docs, and the short-lived BCOPS series, but much of the rest of the understanding of what is appropriate / best practice seems to be undocumented and passed on through cultural diffusion, pointing at some NANOG post from 1998, or vague handwaving towards the cymu secure IOS template…. I was initially excited by Tom's pointing at RFC4271, Sec 9.1.3, which states: "A route SHALL NOT be installed in the Adj-Rib-Out unless the destination, and NEXT_HOP described by this route, may be forwarded appropriately by the Routing Table." This sounded perfect, and I could beat my friend around the head with it… but reading further reveals: "Route aggregation and information reduction techniques (see Section 9.2.2.1) may optionally be applied. Any local policy that results in routes being added to an Adj-RIB-Out without also being added to the local BGP speaker's forwarding table is outside the scope of this document." W
There are also some more inventive ways of getting routes into BGP, like using ExaBGP as an example.
Sure, but using ExaBGP really amounts to originating the prefix from another router. This discussion was (at least theoretically) about local origination on the router in question.
Owen
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/