Re: maximum ipv4 bgp prefix length of /24 ?

[Joshua Miller <contemno () gmail com>]

Seems like we've reached the limits of apriori speculation. At this point
I'd like to see data demonstrating that it's at least viable from a
statistical perspective.

If someone is motivated to demonstrate this, a "backtest" against
historical data would be the next step. Later, one could design the study
to reveal "transient degradation" (loops, drops, etc.) and their
probability, though the duration would be more a function of the
implementation. It would be best to "backtest" the status quo as a control
because it too has transient degradation, for a more apples-to-apples
comparison.

I'm not sufficiently motivated (nor knowledgeable in statistics) to take
this on. I see this more in the domain of vendors to determine the best
approach for their implementation.

On Mon, Oct 2, 2023 at 9:21 AM tim () pelican org <tim () pelican org> wrote:

> On Monday, 2 October, 2023 09:39, "William Herrin" <bill () 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.