Re: Multicast RP - RPF concept

Guys,

I must have messed with the wording to sound like asking about RPF check on
a unicast packet , but thats definitely not what I meant..Here is the
output of the test I carried.. Since I am pasting it out of the test I was
doing the hostnames are different..In this case R6 is the source connected
to R4 and R4 is connected to R5 through two links ...I am getting an RPF
failed on R4 which is not the RP ..

SHOW IP MROUTE BEFORE THE PING
R6#sh ip mr 224.20.20.20
Group 224.20.20.20 not found

R4#sh ip mr 224.20.20.20
Group 224.20.20.20 not found

R5#sh ip mr 224.20.20.20
<output ommitted >

(*, 224.20.20.20), 06:37:30/00:03:29, RP 150.1.5.5, flags: S
  Incoming interface: Null, RPF nbr 0.0.0.0
  Outgoing interface list:
    FastEthernet0/0, Forward/Sparse, 05:44:48/00:03:29
 PING INITIATED FROM R6 and a debug on R4

R6#ping 224.20.20.20
Type escape sequence to abort.
Sending 1, 100-byte ICMP Echos to 224.20.20.20, timeout is 2 seconds:
.
R6#

R4#debug ip mpack deta
IP multicast packets debugging is on (detailed)
R4#
*Jun 23 10:15:17.047: IP(0): MAC sa=000d.bc35.5f60 (FastEthernet0/1), IP
last-hop=155.1.146.6
*Jun 23 10:15:17.047: IP(0): IP tos=0x0, len=100, id=16, ttl=254, prot=1
*Jun 23 10:15:17.047: IP(0): s=155.1.146.6 (FastEthernet0/1) d=224.20.20.20
id=16, ttl=254, prot=1, len=114(100), RPF lookup failed for source or RP
R4#
R4#un all
All possible debugging has been turned off
R4#sh ip rpf 150.1.5.5
RPF information for ? (150.1.5.5) failed, no route exists
R4#
R4#sh ip rpf 155.1.146.6
RPF information for ? (155.1.146.6)
  RPF interface: FastEthernet0/1
  RPF neighbor: ? (155.1.146.6) - directly connected
  RPF route/mask: 155.1.146.0/24
  RPF type: unicast (connected)
  RPF recursion count: 0
  Doing distance-preferred lookups across tables
R4#
As you can see R4 is not failing the RPF check for the source rather the RP
IP address .. A debug ip packet detail on R4 and R5 also shows that there
are actually no packets sent from R4 to R5 , and the RPF is not failing on
the RP .. What I meant in my question was R4 (R2 originally in the question
) knows the RP address so when it receives a register message to be sent to
the RP does it first do a RPF check for the RP address.. and if yes,, does
it do that without actually receiving any packet from the RP unlike in other
scenarios where an RPF check is performed only when a multicast packet is
received..

Hope this time the wording is clear ;-)

Regards,
Ravi

On Tue, Jul 7, 2009 at 4:07 PM, Jared Scrivener <jscrivener_at_ipexpert.com>wrote:

> 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 <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
> >
> >
> > Blogs and organic groups at http://www.ccie.net
> >
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> >
> >
> >
> >
> >
> >
> >
> >
>
>
> --
> Pavel Bykov
> ----------------
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Received on Tue Jul 07 2009 - 16:45:00 ART