Setting sensible max-prefix limits

[Lars Prehn <lprehn () mpi-inf mpg de>]

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?

Apart from your peers, do you also apply a limit to your transit sessions?

Best regards,

Lars