Subject: RE: [OSPF] Correlating unnumbered point-to-point interfaces in OS PFv2

From: "Pandian, Vijay" <[email-masked]>
Date: Tue, 2 May 2006 09:06:55 -0400

Dave,

Thanks for the response and the suggestions.

The reason behind the question is to address an issue in the route
calculation by R1. It is possible that R1 would use L1 in its forwarding
table when R2 is the next-hop. Thus we end up in a black-hole up until the
dead timer expires.

Looks like this has been a known issue (response from Alia Atlas) and BFD or
a more aggressive dead timer probably are the only solutions.

Best regards,

Vijay

> -----Original Message-----
> From: Dave Katz [mailto:[email-masked]]
> Sent: Monday, May 01, 2006 2:33 PM
> To: Pandian, Vijay
> Cc: [email-masked]
> Subject: Re: [OSPF] Correlating unnumbered point-to-point 
> interfaces in
> OSPFv2
> 
> 
> 
> On May 1, 2006, at 9:17 AM, Pandian, Vijay wrote:
> 
> > 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.
> 
> 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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