maximum ipv4 bgp prefix length of /24 ?

Mon Oct 2 07:56:41 UTC 2023

On Sun, Oct 1, 2023 at 9:55 PM Jakob Heitz (jheitz) <jheitz at cisco.com> wrote:
> my main point
> is that FIB compression does not allow you to install a FIB with less memory.

Hi Jakob,

The math disagrees. It's called "oversubscription," and we use it all
over the place in network engineering.

There are only a handful of route patterns that'd result in no
compression at all. They'd have to be intentionally created, and
that'd be a hacking challenge in and of itself. The patterns in
question don't align with the distribution of addresses on the
Internet.

If you're at 80% FIB after compression, a compression transient could
plausibly bump you to 85%. The odds of a natural transient bumping you
to 100% are infinitesimal. If you try to run at 95% after
compression... well, I'm sure someone will try it, but that's PEBKAC
not compression's fault.

FIB compression ranges from 30% in simple core scenarios to more than
90% in edge scenarios with advanced compression. Even keeping
reasonable slack for transients, you're going to get some bang for
your buck. All it means is that you have to keep an eye on your FIB
size as well, since it's no longer the same as your RIB size.

Regards,
Bill Herrin

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