Prefix List - Twisted Logic

f0rgiv3n

OK so I know that if you want to encompass all with a prefix list you use:

0.0.0.0/0 le 32


While this makes complete sense in my mind, it also brought up another thought. Could we do the exact opposite and accomplish the same thing as well? This might be showing my ignorance with this but I have yet to be able to wrap my head around why this SHOULDN'T work:

what if you used:

255.255.255.255/32 ge 0

Is that even possible? Just looking for another set of eyes... I'm sure the reason why it won't work is somewhere, I just am not finding it right now

f0rgiv3n

Alright, so nevermind on this one... My logic doesn't align with what Cisco allows you to do in prefix-lists. I quickly threw together a BGP network and attempted to do this:

Problem#1 - You cannot set the value after the LE or GE to 0, it has to be 1-32
Problem#2 - Even if you do set the value to 1, it won't allow you to set it to 255.255.255.255/32. The error you get when swapping this is:
% Invalid prefix range for 255.255.255.255/32, make sure: len < ge-value <= le-value


So it looks like your initial prefix length has to always be less than the second prefix value... Shucks

Trifidw

f0rgiv3n wrote: »

So it looks like your initial prefix length has to always be less than the second prefix value... Shucks

Nods. In the OCG there is a nice diagram illustrating the permitted ranges.