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Comments on draft-ietf-rtgwg-ipfrr-spec-base-09

Subject: Comments on draft-ietf-rtgwg-ipfrr-spec-base-09
From: mike shand
Date: Fri, 12 Oct 2007 13:05:45 +0100
I support the progression of this document, but would like to see the comments Stewart and I made earlier on the algorithm addressed and also the few points below.
In addition, I wonder whether there should be something up front which
makes clear that this may only provide a partial solution (depending on
the topology). I know this is implicit in the body of the text, but I
think it would be useful to avoid any misunderstandings if the abstract
were a little more explicit on this point.
How about adding after "...The goal of this technology is to reduce the
  micro-looping and packet loss that happens while routers converge
  after a topology change due to a failure. ..."

"The extent to which this goal can be met by this specification is dependant on the topology of the network."
   Mike



   1.1.  Failure Scenarios

      The alternate next-hop can protect against a single link failure, a
      single node failure, one or more shared risk link group failures, or
      a combination of these.


It might be better to say "failure of one or more links within a shared risk link group".
         Figure 5: Example where Continued Use of Alternate is Desirable

      This is an example of a case where the new primary is not a loop-free
      alternate before the failure and therefore may have been forwarding
      traffic through S. This will occur when the path via a previously
      upstream node is shorter than the the path via a loop-free alternate
      neighbor.  In these cases, it is useful to give sufficient time to
      ensure that the new primary neighbor and other nodes on the new
      primary path have switched to the new route.


I wonder if it should be pointed out that while this is a good strategy to minimize the occurrence of microloops, it does nothing to prevent any microloops which may occur more than one hop away.
      based on the new network topology.  The use of the alternate next-
      hops for packet forwarding SHOULD terminate:

      a.  if the new primary next-hop was loop-free prior to the topology
          change, or

      b.  if a configured hold-down, which represents a worst-case bound on
          the length of the network convergence transition, has expired, or

      c.  if notification of an unrelated topological change in the network
          is received.

We should probably add that if the primary link comes back before any of this has happened then you can just go back to using the primary link as if nothing had happened. That of course pre-supposes that the failure hadn't yet been advertised. If it HAS been advertised, then it requires another advertisement to put it back how it was, but in any case (I think) the old next hop can safely be used.

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