Setting sensible max-prefix limits

Wed Aug 18 09:55:38 UTC 2021

On Wed, 18 Aug 2021 at 11:33, Lars Prehn <lprehn at mpi-inf.mpg.de> wrote:
>
> As I understand by now, it is highly recommended to set a max-prefix
> limit for peering sessions. Yet, I can hardly find any recommendations
> on how to arrive at a sensible limit.
>
> I guess for long standing peers one could just eyeball it, e.g., current
> prefix count + some safety margin. How does that work for new peers? Do
> you negotiate/exchange sensible values whenever you establish a new
> session? Do you rely on PeeringDB (if available)? Do you apply default
> values to everyone except the big fishes?

- review max prefix suggestions from the peer itself, either from the
email or peeringdb
- check actual current prefix count (bgp.he.net et all)
- check whether the disparity between the two matches your expectation
of a safety margin, based on your own operational experience and
context
- defaults for low prefix count peers
- actually monitor warning/critical levels of max-prefix counts

Don't use too small a safety margin, you don't want to spend your days
adjusting max-prefix levels all the time.

I don't have strict rules for the safety margin itself; it depends
very much on the network (size, growing rate, trust, history).

lukas