Subject: [OSPF] Correlating unnumbered point-to-point interfaces in OSPFv2

From: "Pandian, Vijay" <[email-masked]>
Date: Mon, 1 May 2006 12:17:12 -0400

Hi,

Assume there are two routers, R1 and R2, connected by two unnumbered
point-to-point interfaces, say L1 and L2. Assume also that the OSPF
adjacency is established over these two links and are listed in the Router
LSA's originated by R1 and R2. 

At a later point in time, L1 experiences a unidirectional failure such that
R2 detects the failure but not R1. R2 now re-originates its Router LSA with
L1 removed from the list of links. 

When R1 receives the new Router LSA from R2, how would it realize that L1 is
down - before the RouterDeadInterval. Since both routers advertise only
their local MIB-II IfIndex value in the  Link Data filed, how would each
routers correlate the interfaces. 

Is this an implementation specific issue? or is there additional information
available in the protocol that can be used to resolve this?
  
Regards,

Vijay

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From: Dave Katz <[email-masked]>
Date: Mon, 1 May 2006 11:33:10 -0700

> routers correlate the interfaces.

In general, you're hosed until the adjacency is seen as being down on  
both sides.  This is inherent to the protocol.  There is an  
assumption that a failure will cause a bidirectional adjacency  
failure in an acceptable amount of time.  If waiting for the dead  
timer to expire is too long, then you need a shorter dead timer (or  
use BFD.)

Your question implies that you're trying to divine adjacency state by  
looking at the LSA database;  if so, this is a bad idea, since the  
database may not be up to date and may be in flux.

--Dave

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From: "Alia Atlas" <[email-masked]>
Date: Mon, 1 May 2006 12:00:52 -0700

> available in the protocol that can be used to resolve this?

If there is only one unnumbered interface between R1 and R2, then it
is easy to match them up.  Otherwise, there is a link-local extension
in the GMPLS work, as I recall, that provides the capability to match
up the unnumbered interfaces.  The issue comes up with IP FRR as well,
because it is necessary/desirable to correctly match up the two
uni-directional links so that the potential failures are considered.

Alia