Subject: Re: [pim] a problem about switching to SPT

From: John Zwiebel <[email-masked]>
Date: Tue, 24 Sep 2002 09:27:32 -0700

"Nescafe" <[email-masked]> wrote:

Thank you very much. I will follow your advice.
But I still doubt that if SPT and WRT are only partly the same
as each other just near the last hop router, in fact they branch at some
other point. DO we should switch to SPT?I think it's more reasonable, but
who will send (S, G) join first?

-------------------

Setting the T-bit and joining to the SPT are two different operations.
The last-hop router (except if you go out of your way to prevent it)
 -always- joins to the SPT.

The T-bit only needs to be set when the RPF neighbor toward the source
 is different than the RPF neighbor toward the RP.

Having said that, an implementation -may- choose to set the T-bit
 immediately even though the RPF neighbor for both paths is the
 same.  Setting the T-bit means the router will send (S,G,R) prunes
 toward the RP.  If the upstream neighbor has implemented
 'inherited/immediate' OIFs as outlined in the new PIM draft,
 then it will ignore those (S,G,R) prunes because the last-hop
 would have also sent an (S,G) join making that OIF 'immediate'.

I'm not saying that it is correct for the last-hop to do this, I'm
 saying that if all implementations in the network are  compliant
 with the new spec, it won't make a difference in the forwarding of
 mpackets.

Of course, if the RPT and the SPT diverge, the router where this
 happens must prune off the source from the shared-tree, meaning
 it must send (S,G,R) prunes.

So, in case I'm not clear, your questions were:
  1) Do we switch to the SPT?
      Yes, the last-hop router is responsible for switching to
      the SPT.

  2) Who will send (S,G) join first?
        The last-hop router

_______________________________________________
pim mailing list
[email-masked]
http://catarina.usc.edu/mailman/listinfo/pim