Re: maximum ipv4 bgp prefix length of /24 ?

[William Herrin <bill () herrin us>]

On Fri, Sep 29, 2023 at 12:54 PM Collider <large.hadron.collider () gmx com> wrote:
> This thread is utter amateur hour. I too would rather /32s be valid in the DFZ - but they're not, for good reason 
> (worstcase scenario = circa 4 bln. routing table entries - no BGP hwaccel can swing that!).

Howdy,

Actually, BGP can swing that. Routing involves two distinct
components: the routing information base (RIB) and the forwarding
information base (FIB). BGP is part of the RIB portion of that
process. It's always implemented in software (no hardware
acceleration). It's not consulted per-packet, so as long as the update
rate is slow enough for the CPU to keep up and there's enough DRAM
(which is cheap and plentiful these days) to hold the entire thing,
there's no particular upper bound to the number of routes.

The router then processes all of the RIBs for all of the routing
protocols used on the router to find the best routes for any given
destination and match them to the next hops to which packets should be
sent. The results are stored in the FIB. This FIB is what's consulted
for each packet passing through the router.

The limiting factor is the FIB. The FIB is what is implemented with
hardware acceleration on high-end routers in order to achieve large
packet-per-second (PPS) numbers. It relies on storage hardware which
is both faster and more expensive than DRAM. Consequently it has much
less capacity to store information than DRAM. Currently shipping
equipment intended for BGP backbone use can manage 1M to 2M routes in
the hardware-accelerated FIB regardless of the amount of DRAM on the
machine.

Regards,
Bill Herrin


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