"Tactical" /24 announcements

Thu Aug 12 17:19:44 UTC 2021

On Thu, Aug 12, 2021 at 9:41 AM Hank Nussbacher <hank at interall.co.il> wrote:
> On 12/08/2021 17:59, William Herrin wrote:
> > If you prune the routes from the Routing Information Base instead, for
> > any widely accepted size (i.e. /24 or shorter netmask) you break the
> > Internet.
>
> How does this break the Internet?  I would think it would just result in
> sub-optimal routing (provided there is a covering larger prefix) but
> everything should continue to work.  Clue me in, please.

A originates 10.0.0.0/16 to paid transit C
B originates 10.0.1.0/24 also to paid transit C
C offers both routes to D. D discards 10.0.1.0/24 from the RIB based
on same-next-hop
You peer with A and D. You receive only 10.0.0.0/16 since A doesn't
originate 10.0.1.0/24 and D has discarded it.
You send packets for 10.0.1.0/24 to A (the shortest path for
10.0.0.0/16), stealing A's paid transit to C to get to B.
Unless A filters C-bound packets purportedly from 10.0.1.0/24. B
doesn't currently transit for A so from B's perspective that's not an
allowed path. In which case, your path to 10.0.1.0/24 is black holed.

D broke the Internet. If packets from you reach A at all, they do so
through an unpermitted path.

Regards,
Bill Herrin

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