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

From: Nitin Kakkar <[email-masked]>
Date: Mon, 20 Feb 2006 12:20:48 +0530

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)

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.

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.
--Nitin

-----Original Message-----
From: Mailing List [mailto:[email-masked]] On Behalf Of Dave
Katz
Sent: Monday, February 20, 2006 11:41 AM
To: [email-masked]
Subject: Re: Detecting router id change in p2p-over-lan connection

On Feb 19, 2006, at 9:32 PM, Nitin Kakkar wrote:

> Sunil,
>   Why do you want to detect router-id change on P2P link ?
>   I have seen some major implementations which can support multiple
> simultaneous adjacencies on P2P link. So what is the meaning of
> router-id change?

Such implementations are out of spec, not to mention that it is not  
at all clear how one would address multiple devices on a point-to- 
point link.

By definition a point-to-point link connects exactly two devices;  if  
there are more than two then it is by definition multipoint.

Unless you are referring to multiple adjacencies between the same  
pair of routers, which is a whole separate can of worms.

Detecting router ID changes is important in order to detect cable  
moves, or a router ID change, or some kind of system hot swap (such  
as APS) among other things.

--Dave

---

From: Dave Katz <[email-masked]>
Date: Mon, 20 Feb 2006 19:05:31 -0800

> 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.

> 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.

> 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