Re: maximum ipv4 bgp prefix length of /24 ?

Mon Oct 2 13:19:27 UTC 2023

On Monday, 2 October, 2023 09:39, "William Herrin" <bill at herrin.us> said:

> That depends. When the FIB gets too big, routers don't immediately
> die. Instead, their performance degrades. Just like what happens with
> oversubscription elsewhere in the system.
> 
> With a TCAM-based router, the least specific routes get pushed off the
> TCAM (out of the fast path) up to the main CPU. As a result, the PPS
> (packets per second) degrades really fast.
> 
> With a DRAM+SRAM cache system, the least used routes fall out of the
> cache. They haven't actually been pushed out of the fast path, but the
> fast path gets a little bit slower. The PPS degrades, but not as
> sharply as with a TCAM-based router.

Spit-balling here, is there a possible design for not-Tier-1 providers where routing optimality (which is probably not a word) degrades rather than packet-shifting performance?

If the FIB is full, can we start making controlled and/or smart decisions about what to install, rather than either of the simple overflow conditions?

For starters, as long as you have *somewhere* you can point a default at in the worst case, even if it's far from the *best* route, you make damn sure you always install a default.

Then you could have knobs for what other routes you discard when you run out of space.  Receiving a covering /16?  Maybe you can drop the /24s, even if they have a different next hop - routing will be sub-optimal, but it will work.   (I know, previous discussions around traffic engineering and whether the originating network must / does do that in practice...)

Understand which routes your customers care about / where most of your traffic goes?  Set the "FIB-preference" on those routes as you receive them, to give them the greatest chance of getting installed.

Not a hardware designer, I have little idea as to how feasible this is - I suspect it depends on the rate of churn, complexity of FIB updates, etc.  But it feels like there could be a way to build something other than "shortest -> punt to CPU" or "LRU -> punt to CPU".

Or is everyone who could make use of this already doing the same filtering at the RIB level, and not trying to fit a quart RIB into a pint FIB in the first place?

Thanks,
Tim.