Subject: RE: Few questions

From: Mathur Sonum-CSM109 <[email-masked]>
Date: Fri, 29 Jun 2001 13:29:50 -0500

> 
> Did you mean "Prune"?  (S,G,R)JOINs are very rare and only used to
> override
> a prune when you've had a "strange" topology that causes the upstream
> routers
> assert for who the forwarder is.

(S,G,R) Joins are possible even without asserts - when a router sends (S,G,R) Prune on the shared X-line and some other router (who is just on a shared tree) wants to override this (S,G,R) Prune. This other router will send (S,G,R) Join towards RP or the turnaround router (when RP is away case).

--
Sonum

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From: John Zwiebel <[email-masked]>
Date: Fri, 29 Jun 2001 12:08:16 -0700

> (S,G,R) Joins are possible even without asserts - when a router sends (S,G,R) Prune on the shared X-line and some other router (who is just on a shared tree) wants to override this (S,G,R) Prune. T…

I didn't say they weren't possible.  I was trying to understand the
question
being posed since it talked about switching from the RPT to the SPT and
sending an (S,G,R) JOIN.  I think Pawan meant to say that when you
switch
back from the shortest-path to the shared-path.  IMHO, not a good thing
to
plan on doing since it ends up with data paths coming and going and will
result
in duplicate packets that may then cause you to exceed your
spt-threshold so
you switch back to the shortest-path again, only to then flip-flop back
to
the shared-tree again.  If you get lots of receivers doing this your
network
gets hosed.  Your mileage may very.

Depending on your implementation, the only time you should need to send
an (s,g,r)join override is when you have a LAN with two upstream
routers,
one the RPF to the RP the other the RPF to the source, and two
downstream 
routers, one staying on the shared tree, one trying to prune the
shared-tree
(so sending the (s,g,r) prunes that have to be overridden).

If the (s,g,r) prunes are being sent to the upstream router that is the
RPF
to the RP, there's no reason to send an (s,g,r) join to override this
since
the upstream "should have known" that there was a downstream router that
wanted
to stay on the shared-tree and it wouldn't prune this interface.

So, its the "other" upstream router that would be receiving the (s,g,r)
prune
that would have to be overridden.  The only way the "other" router can
become
the RPF is through some kind of assert process so that both downstream
routers
now use RPF' for the (s,g,r).

(s,g,r)JOINs should be a very, very rare occurance.