Subject: question on the hop-limit constraint in One-to-one (detour) fast- reroute

From: Ling Li <[email-masked]>
Date: Wed, 26 Jun 2002 10:58:52 -0400

The hop-limit constraint is defined as follows in 
draft-ietf-mpls-rsvp-lsp-fastreroute-00.txt:

Hop-limit

      The maximum number of extra hops the backup path is allowed
      to take, from current node (a PLR) to a MP, with PLR and MP
      excluded in counting.  For example, hop-limit of 0 means only
      direct links between PLR and MP can be considered.

I assume that the MP here refers to the MP where backup tunnels rejoin 
the path of the protected LSP, not the Detour Merge Point, which is defined
in the same document as:

MP - Merge Point. The LSR where one or more backup tunnels rejoin
          the path of the protected LSP, downstream of the potential
          failure. In the case of one-to-one backup, a Merge Point may
          also be an LSR where multiple detours converge and only one
          detour is signaled beyond that LSR; this type of merge point
          may be referred to as a Detour Merge Point.  A MP may also
          be a PLR. 

Seems to me that an assumption is made here that every node
on the downstream path of the protected LSP should/must be a MP. Otherwise
if some of the nodes are MP and some are not capable of merging LSPs, and a
PLR that computes a detour path has no knowledge of which one is a MP (this
info. is not flooded by ospf/is-is), how could the hop-limit constraint be
applied?

If the above assumption is valid, should the destination of the detour LSP
be
the MP, rather than the final destination of the protected LSP?

Any suggestions would be appreciated.

Thanks, 

Ling Li 

Axiowave Networks, Inc. 
200 Nickerson Road 
Marlborough, MA 01752 
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