Re: "Tactical" /24 announcements

[William Herrin <bill () herrin us>]

On Thu, Aug 12, 2021 at 7:44 AM Tom Hill <tom () ninjabadger net> wrote:
> On 11/08/2021 14:09, Jon Lewis wrote:
> > At least one major network hardware vendor has implemented it as a
> > feature.  Turn it on, and the "deaggregates" with same next-hop as an
> > aggregate are not programmed into the FIB.  The savings will vary
> > depending on the device's connectivity, but I've seen >40%.
>
> Limiting the pruning to cases with the same next-hop does indeed sound
> like it would be safer than what I've seen done in the past.

Hi Tom,

To be clear, Jon was talking about pruning it from the FIB not the
RIB. You can always safely prune overlapping routes with the same next
hop from the Forwarding Information Base because the FIB lookup will
still select the same next hop regardless. This is valuable because
the main cost driver is carrying the routes in the FIB table that's
consulted for every packet handled.

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. Just because it's the same next hop for you doesn't mean the
routes actually share fate. The routers past you need both routes to
figure out their own position in a valid path. And it doesn't save you
much anyway because the RIB is only consulted when routes change and
need to be reprocessed into FIB entries. A $10 virtual server can
handle today's BGP RIB with ease and equipment with only a little more
power could handle one much larger. It's the FIB which drives the
limits.

Regards,
Bill Herrin


-- 
William Herrin
bill () herrin us
https://bill.herrin.us/