Duplicate responses to multicast ping

Unanswered Question
Mar 3rd, 2007

I have a lab with 7 routers running PIM in sparse-dense mode with Auto-RP. Everything seems to work, but on some routers pinging a multicast group address produces more than one response:

R1#ping 239.0.0.1 rep 4

Type escape sequence to abort.

Sending 4, 100-byte ICMP Echos to 239.0.0.1, timeout is 2 seconds:

Reply to request 0 from 10.0.2.12, 1 ms

Reply to request 0 from 10.0.2.12, 80 ms

Reply to request 0 from 10.0.2.12, 80 ms

Reply to request 1 from 10.0.2.12, 1 ms

Reply to request 1 from 10.0.2.12, 80 ms

Reply to request 1 from 10.0.2.12, 80 ms

Reply to request 2 from 10.0.2.12, 1 ms

Reply to request 2 from 10.0.2.12, 80 ms

Reply to request 2 from 10.0.2.12, 80 ms

Reply to request 3 from 10.0.2.12, 1 ms

Reply to request 3 from 10.0.2.12, 80 ms

Reply to request 3 from 10.0.2.12, 80 ms

Here it is 3 responses to each multicast ping. This doesn't look good, but I don't see how does this happen.

"sh ip mroute" for the group looks reasonable, but the "Registering" message looks suspicious (it never disappears)

(*, 239.0.0.1), 00:00:11/stopped, RP 10.0.0.5, flags: SPF

Incoming interface: Serial6/7:0.2, RPF nbr 10.0.1.130

Outgoing interface list: Null

(10.0.1.1, 239.0.0.1), 00:00:11/00:03:24, flags: FT

Incoming interface: Serial6/7:0.1, RPF nbr 0.0.0.0, Registering

Outgoing interface list:

Serial6/7:0.2, Forward/Sparse-Dense, 00:00:11/00:03:24

"debug ip mpacket 239.0.0.1" on an intermediate router sees three packets from the original source router; the IP addresses all belong to the router. It looks like the router sends the pings from all multicast enabled interfaces... why?

Mar 3 21:37:02.701: IP(0): s=10.0.9.1 (FastEthernet0/0) d=239.0.0.1 (FastEthernet0/1) id=2141, ttl=253, prot=1, len=100(100), mforward

Mar 3 21:37:02.781: IP(0): s=10.0.1.129 (Serial0/0/0:0) d=239.0.0.1 (FastEthernet0/1) id=2141, ttl=254, prot=1, len=100(100), mforward

Mar 3 21:37:02.785: IP(0): s=10.0.1.1 (FastEthernet0/0) d=239.0.0.1 (FastEthernet0/1) id=2141, ttl=253, prot=1, len=100(100), mforward

I have this problem too.
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dsweeny Thu, 03/08/2007 - 11:53

seems to be caused by inappropriate link-level retransmissions. Duplicates may occur in many situations and are rarely (if ever) a good sign, although the presence of low levels of duplicates may not always be cause for alarm. Duplicates are expected when pinging a broadcast or multicast address, since they are not really duplicates but replies from different hosts to the same request

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