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RE: PIM Sparse-Mode join RPF question posted 01/04/2006
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The mroute is only an override to the INCOMING multicast stream check.  So
yes, you would need it, because as any information comes from the RP it
would show the wrong interface.  The same may be true of your multicast
source IP (if you had one) for the stream itself.

As for outbound information, the initial conversation to the RP is unicast,
so the path doesn't always matter (that's the simple answer, there are more
complicated reasons with this that may be more than you care to get into!
(the Beau Williamson book from Cisco Press is very good at this)).  If you
run into issues or have multicast in both directions, it may also be
necessary for an ip mroute on R1 that shows R2's information (typically the
multicast source IP whether an ethernet or loopback).

If you want to watch the process and get a much better feel for it, type
doing a "debug ip mpacket detail" and "debug ip mrouting rpf-events" to see
whether things are what you expect or not!



-----Original Message-----
From: nobody@xxxxxxxxxxxxxx [mailto:nobody@xxxxxxxxxxxxxx] On Behalf Of Greg
Sent: Wednesday, January 04, 2006 11:56 AM
To: ccielab@xxxxxxxxxxxxxx
Subject: PIM Sparse-Mode join RPF question

I was hoping one of the resident Multicast experts could explain this
scenario - maybe Mr. Scott Morris? (I read your chapter in CCIE practical

  [R1]-L0 <--- This is the RP
s0| |s1
   | |
s0| |s1 <--- This interface is the preferred unicast route to RP but PIM 
s0| |is

R1 and R2 are connected to each either by two point to point T1 links.
R2 needs to send a pim sparse-mode join to the RP which is the loopback
address of R1.
R2's unicast routing table shows Serial1 as the best route to the RP. 
Unfortunately PIM is not allowed to be enabled on Serial1 and is only
enabled on Serial0.

In this situation, would the PIM join then be sent via Serial 0 instead? Or
would this cause an RPF failure?

I'm thinking the join would not be forwarded.
I believe the solution was to add a quad zero mroute pointing to the IP of
R1's Serial 0 interface.

Could someone explain why that would worK?

Thanks in advance!

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