Re: maximum ipv4 bgp prefix length of /24 ?

[Owen DeLong via NANOG <nanog () nanog org>]

> On Oct 2, 2023, at 01:18, Nick Hilliard <nick () foobar org> wrote:
>
> William Herrin wrote on 02/10/2023 08:56:
> > All it means is that you have to keep an eye on your FIB
> > size as well, since it's no longer the same as your RIB size.
>
> the point Jacob is making is is that when using FIB compression, the FIB size depends on both RIB size and RIB 
> complexity.  I.e. what was previously a deterministic 1:1 ratio between RIB and FIB - which is straightforward to 
> handle from an operational point of view - becomes non-deterministic. The difficulty with this is that if you end up 
> with a FIB overflow, your router will no longer route.

It was never 1:1 if you have more than one path for any route. The FIB only contains the chosen path(s) to any 
destination even without fib compression. 

> That said, there are cases where FIB compression makes a lot of sense, e.g. leaf sites, etc. Conversely, it's not a 
> generally appropriate technology for a dense dfz core device.  It's a tool in the toolbox, one of many.

Even at a dense DFZ core device, there are a large number of single-origin consecutive /24s in the table which can be 
fib compressed with no loss. For a long time, someone was teaching up and coming operators in Asia that they should 
always announce everything as disaggregated /24s to guard against route hijacking. This unfortunate practice persists 
still, making FIB compression quite practical even at core nodes. 

Owen

> Nick