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/ >
