RE: Multicast RP - RPF concept

This also explains the odd behavior seen occasionally when pinging a
multicast address where the first ping packet responds but the subsequent
ones fail.

Cheers,
 
Jared Scrivener CCIE3 #16983 (R&S, Security, SP), CISSP
Sr. Technical Instructor - IPexpert, Inc.
URL: http://www.IPexpert.com
Telephone: +1.810.326.1444
Fax: +1.810.454.0130
Mailto: jscrivener_at_ipexpert.com

-----Original Message-----
From: nobody_at_groupstudy.com [mailto:nobody_at_groupstudy.com] On Behalf Of
Pavel Bykov
Sent: Tuesday, 7 July 2009 9:16 AM
To: Ravi Singh
Cc: Cisco certification
Subject: Re: Multicast RP - RPF concept

As Lejoe pointed out, there are no checks on R2 since it's UNICAST, so it
behaves as unicast in all manners. The problem in your case are two links
towards R3. register would arrive at R3, R3 would decapsulate the packet,
and then do the RPF - that's where RPF would fail, and that's where the
counters would increment.

On Tue, Jul 7, 2009 at 1:15 PM, Ravi Singh <way2ccie_at_googlemail.com> wrote:

> Hi Group,
>
> Just trying to clarify a bit in the RPF check mechanism in PIM Sparse
> mode..Suppose 3 routers R1, R2 and R3 are connected linearly .. with R1 as
> the source of the multicast and R3 as the RP.. Not concerned with the
> receivers in this example.. If R1 sends traffic towards the receiver
> (downstream R3) , the unicast register reaches R2 and R2 does a RPF check
> on
> the source of the traffic i.e R1 as well as the RP i. R3 .. Now as I have
> read it when a router receives a multicast packet it does a RPF check on
> the
> source of the packet. But in this case the RP (i.e R3 ) has not yet sent
> any
> traffic towards R2 so R2 has not received any packet from the RP.. Yet R2
> does a check on the RP address.. My question is have I understood this
> correctly that the RPF check for the RP address is done without actually
> any
> multicast traffic being received from the RP ...while for other scenarios
> the multicast packet has first to be received from a source and then the
> RPF
> check is performed ..
>
> I have a scenario where R2 has two connections to R3 (RP ) and one of the
> connections has PIM enabled while the RP is reachable through the other
> connection ..Essentially, the RPF check on R2 fails for the RP and when I
> debug the packets there is no packet which was sent to the RP .. the check
> failed on R2 straightaway ...
>
> Hope I have put the question properly !!
>
> Regards,
> Ravi
>
>
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-- 
Pavel Bykov
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