Source specific multicast does not work--how can I debug?

Unanswered Question
Mar 5th, 2007

I have small lab network where I want to try Source Specific Multicast. Setup is very basic and according to Cisco documentation. Alas, it for some reason doesn't work. I am trying to find out why. (I am preparing for CCIE lab, by the way).

The source->receiver path is SW1-R4-R5

On R5 there is interface Loopback0, that joins group

interface Loopback0

ip address

ip pim sparse-mode

ip igmp join-group source

ip igmp version 3

All routers have "ip pim ssm default" in their configs; "ip pim sparse-mode" is configured on all interfaces. In fact, normal PIM SM works on the same setup fine (of course, RPs need to be added).

I try to ping from SW1:

SW1#ping source repeat 4

Sending 4, 100-byte ICMP Echos to, timeout is 2 seconds:

Packet sent with a source address of


Doesn't work, so let's go from receiver to source:

R5#sh ip mroute

(,, 02:56:25/00:02:47, flags: sLTI

Incoming interface: FastEthernet0/0, RPF nbr

Outgoing interface list:

Loopback0, Forward/Sparse, 02:56:25/00:02:47

R4#sh ip mroute

(,, 03:28:55/00:02:54, flags: sT

Incoming interface: FastEthernet0/1, RPF nbr

Outgoing interface list:

FastEthernet0/0, Forward/Sparse, 02:57:00/00:02:54

SW1#sh ip mroute

(,, 03:29:55/00:03:23, flags: sT

Incoming interface: Null, RPF nbr

Outgoing interface list:

Vlan2, Forward/Sparse, 02:58:01/00:03:23

This looks good, the IP addresses in "RPF nbr" fields are OK, the interfaces are OK as well.

I tried "debug ip mpacket" on R5 and R4, but that doesn't show anything, like no multicast traffic is ever flowing.

What else can I try to solve this problem? What can even go wrong with such a simple setup.

I have this problem too.
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Harold Ritter Mon, 03/05/2007 - 11:46

The "show ip mroute" outputs look good. Silly question but does R5 have a unicast route back to


borek Mon, 03/05/2007 - 12:09

Yes, I have checked the connectivity thoroughly (or so I hope):

R5#sh ip ro

Routing entry for

Known via "bgp 2", distance 200, metric 0

Tag 3, type internal

Last update from 1d03h ago

Routing Descriptor Blocks:

*, from, 1d03h ago

Route metric is 0, traffic share count is 1

AS Hops 1

Route tag 3


Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to, timeout is 2 seconds:


Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms

Harold Ritter Mon, 03/05/2007 - 15:54

Try applying the "ip igmp join-group source" command on R5 FastEthernet0/0 instead of loopback0.

Hope this helps,


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