Re: maximum ipv4 bgp prefix length of /24 ?

[William Herrin <bill () herrin us>]

On Fri, Sep 29, 2023 at 3:26 PM Owen DeLong <owen () delong com> wrote:
> > On Sep 29, 2023, at 15:14, William Herrin <bill () herrin us> wrote:
> > I'm less assuming it and more reading it from this SIGCOMM paper:
> > https://people.csail.mit.edu/ghobadi/papers/trio_sigcomm_2022.pdf
>
> Fair enough, but interestingly, I think that the compiled line-card forwarding
> table probably always fits in the cache.

If I were designing the product, I'd size the SRAM with that in mind.
I'd also keep two full copies of the FIB in the outer DRAM so that the
PPEs could locklessly access the active one while the standby one gets
updated with changes from the RIB. But I'd design the router to
gracefully fail if the FIB exceeded what the SRAM could hold.

When a TCAM fills, the shortest prefixes are ejected to the router's
main CPU. That fails pretty hard since the shortest prefixes tend to
be among the most commonly used. By comparison, an SRAM cache tends to
retain the most commonly used prefixes as an inherent part of how
caches work, regardless of prefix length. It can operate close to full
speed until the actively used routes no longer fit in the cache.

Regards,
Bill Herrin



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