Subject: when to run in NBMA or P2MP mode?

From: Dhaval Shah <[email-masked]>
Date: Mon, 30 Sep 2002 17:28:24 -0700

Hi
        According to RFC 2328, Ospf can run in one of the two modes on a
non-broadcast networks, NBMA or P2MP. RFC says that you can run in NBMA mode
if all routers attached to the network can communicate directly and you should
run in P2MP mode if thats not possible.
        However, while sending hellos out on non-broadcast networks, irrespective of
which mode you run on, you always send them unicast to your neighbors.
        The essential difference between NBMA and P2MP is only that there is DR/BDR
election and Network LSA generation in NBMA mode and not in P2MP mode.
        So why is the need for the restriction that one MUST be able to reach all
neighbors directly in order to run in NBMA mode? What are some of the example
physical networks where one would like to run in P2MP mode and not in NBMA
mode? How can one determine which mode to run when?

thanks
Dhaval.