On Wed, Jan 31, 2024 at 5:20 PM, Tom Beecher <beec...@beecher.cc> wrote:
> 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 :) ) > Sweet! That seems exactly like what I need — I'll go rub his nose in it (what else are friends for?) Thank you. W > 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 <war...@kumari.net> wrote: > >> >> >> >> >> On Wed, Jan 31, 2024 at 3:56 PM, William Herrin <b...@herrin.us> wrote: >> >>> On Wed, Jan 31, 2024 at 12:30 PM Warren Kumari <war...@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 >>> b...@herrin.us >>> https://bill.herrin.us/ >>> >>
