maximum ipv4 bgp prefix length of /24 ?

Sat Sep 30 06:59:54 UTC 2023

On 9/29/23 22:56, William Herrin wrote:

> 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.

Not unless you are running BGP Add-Paths in its extreme guise to 
consider all available paths, and then there is the chance you could 
push your RAM and CPU into unknown territory, depending on the platform, 
code and control plane you've got.

> 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.

There are line cards out there today that are able to hold 10 million 
routes in FIB.

Mark.
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