I am looking for technical clarity on why someone should / shouldn’t use ISIS p2p topologies on Ethernet media.
Ethernet is a broadcast medium , however if it’s configured as a p2p link with /31’s the broadcast domain is minimised , therefore as I understand it the advantages are the reduction in the amount of information routing protocols must carry and manage and DR/DIS election can be omitted. ( ref here draft-ietf-isis-igp-p2p-over-lan
I am able to find little evidence to support this theory in practice therefore I am looking for sound technical reasoning to justify the above and ideally some information on how this affects convergence.
Best Answer
I configure all my core OSPF interfaces as p2p. I would do the same for integrated IS-IS. This stops the DR/DIS election process from happening which can take a fair amount of time in a re-convergence scenario.
You do not HAVE to have them running as /31. It could be any mask as long as you only have two active speakers.
An added advantage on the OSPF side is that you get no type2 generated for the link.
From Cisco's site as well in regards to integrated IS-IS (http://www.cisco.com/en/US/docs/ios/12_2s/feature/guide/fissp2p.html#wp1037322)