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. 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/
