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Re: Detecting router id change in p2p-over-lan connection

On Feb 19, 2006, at 10:50 PM, Nitin Kakkar wrote:

IMHO OSPF spec does not say that P2P adjacencies should be restricted to
only one peer. (But I haven't read rfc for long time so if does say that
some where please let me know) 

From the spec:

        Point-to-point networks
            A network that joins a single pair of routers.  A 56Kb
            serial line is an example of a point-to-point network.

   The vertices of the graph consist of routers and
        networks.  A graph edge connects two routers when they are
        attached via a physical point-to-point network.

   Lines between routers indicate point-to-point
        networks.

Etc.  It is completely fundamental to the way the protocol works.


How this is possible, Well have two routers with OSPF adjacency and
large dead interval. Change router-id on one of them. ( Note that some
implementations do send one way hello for router-id change they will
behave differently but again ospf spec does not mention that either) but
others will show 2 adjacencies till one of the peer ages out.

Ah, OK, I thought you were saying that you could have adjacencies to two different systems.

Having said that, there is still a fundamental assumption here, namely that the new system supercedes the old at the time when the new adjacency comes up (the local router must not advertise adjacency to the old router ID.) The local router can *not* make the assumption that the new adjacency is to the same system, and in fact the local router has no way of ever knowing, or there are all kinds of really ugly black hole scenarios. There is effectively only one adjacency, regardless of what the "show" command says.


IMHO On P2P cable move should trigger Loss of signal->link-down/ link-up
events which inturn is IFSM_DOWN-> KILL NBR. In this case there won't be
any Nbr history. But if there is a L2 device in between peer then yes
there wont be any link down event.

You cannot assume that there will be a link down event. It obviously helps speed things along (BFD serves a similar purpose.) Particularly in the LAN environment, the odds of a link down event are slim.

--Dave