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

Wed Jan 31 22:08:18 UTC 2024

On Wed, Jan 31, 2024 at 1:46 PM Warren Kumari <warren at kumari.net> wrote:
> On Wed, Jan 31, 2024 at 3:56 PM, William Herrin <bill at herrin.us> wrote:
>> On Wed, Jan 31, 2024 at 12:30 PM Warren Kumari <warren at kumari.net> wrote:
>> 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…

Hi Warren,

I'm not sure what you mean. Aggregation means that if at least one
more-specific is present, the aggregate will be announced. If none of
the more-specifics are present, the aggregate will not be announced.
If you have a default route, I suppose you could end up with a loop,
but that would be your fault for failing to tie down the route you
were announcing. Another reason why it's best practice to have that
explicit route to discard. If you don't have a default route,
recognize that there is an -implicit- discard route for default which
catches everything for which you do not have a route.

Regards,
Bill Herrin

-- 
William Herrin
bill at herrin.us
https://bill.herrin.us/