Question about Multicast-routing Tunnel interface behavior

[Suzukikoki]

The following settings are made so that interface Loopback0 can forward multicast packets.

interface Loopback0
ip address X.X.X.X X.X.X.X
ip mtu 1500
ip pim query-interval 2
ip pim sparse-mode

This setting automatically creates a tunnel interface, but when we actually checked the status of the tunnel interface, we found that traffic was not being forwarded.

#show interface Tunnel0
Tunnel0 is up, line protocol is up
Hardware is Tunnel
Description: Pim Register Tunnel (Encap) for RP X.X.X.X

------omission

5 minute input rate 0 bits/sec, 0 packets/sec
5 minute output rate 0 bits/sec, 0 packets/sec
0 packets input, 0 bytes, 0 no buffer
Received 0 broadcasts (0 IP multicasts)
0 runts, 0 giants, 0 throttles
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored, 0 abort
0 packets output, 0 bytes, 0 underruns
Output 0 broadcasts (0 IP multicasts)
0 output errors, 0 collisions, 0 interface resets
0 unknown protocol drops
0 output buffer failures, 0 output buffers swapped out

Am I correct in understanding that the Tunnel interface only encapsulates the PIM registers and does not actually forward traffic, resulting in this value?
Also, is my understanding correct that the interface that actually forwards traffic is not the Tunnel interface, but theinterface with

ip pim sparse-mode

set?

[Harold Ritter]

Hi @Suzukikoki ,

This tunnel is indeed used to encapsulate the PIM register messages between the first hop router (multicast router connected to the source) and the RP. The first PIM register message also carries the use data (actual multicast message sent by the source). The following PIM register messages are empty and are used to refresh the (S,G) state on the RP. It looks like the tunnel interface counter (packets output) is incremented only for the first PIM register message and not for the following empty PIM register messages. 

Regards,

[Suzukikoki]

Hi @Harold Ritter 

Thank you for your detailed explanation.
I think I understand.

In other words, am I correct in assuming that the counter value of this auto-generated interface will not be increased?

[Harold Ritter · Accepted solution]

Hi @Suzukikoki ,

As I mentioned in my previous post, the

packets output

counter should be incremented for the first packet PIM register messages for a given stream that also contains user data. From what I have seen, it will not be incremented for the following PIM register messages (null registers), which do not contain user data.

Regards,

[Suzukikoki]

Thank you very much for your response.

Understood!

[Harold Ritter]

You are very welcome @Suzukikoki and thanks for the feedback