Make PIMv2 Hello from C1921 backwards-compatible with old C3620 router?

[richard-artevea]

I observed that when I was trying to troubleshoot a field issue using two routers I had in the lab, that one of them (an ancient C3620) did not like the PIMv2 Hello messages sent to it by the newer one (C1921).

The symptom observed with multicast routing is that when a source connected to the C1921 tries to register with an RP running on the C3620, the multicast route stays in the "registering" state forever. But going "the other way" - with the source connected to the C3620 and the RP running on the C1921 worked fine.

Here's what I see in the C1921 when it tried to register a connection to the group 224.0.0.39 (for Auto-RP). Note: The two routers are connected via a GRE tunnel.

(192.168.210.224, 224.0.1.39), 17:09:02/00:03:20, flags: LFT

  Incoming interface: Loopback0, RPF nbr 0.0.0.0, Registering

  Outgoing interface list:

    Tunnel10, Forward/Sparse-Dense, 17:09:02/00:02:53

PIM debugging on the C3620 showed the following when the two exchanged the Hello messages

18:21:57: PIM: Send v2 Hello on Tunnel0

18:21:57: PIM: Received v2 Hello on Tunnel0 from 172.16.0.2

18:21:57: PIM: Hello packet has unknown option 20, ignored

18:21:57: PIM: Hello packet has unknown option 65004, ignored

I believe that the issue is down to the firmware in the C3620 simply being too old and not understanding the PIM options defined in RFC3973 or the Cisco private PIM option 65004 being sent by the C1921 (though the PIM debug does not make it clear whether it is just the unknown options that are ignored or the whole Hello message)

Is there any global or interface-specific PIM option I can use in the C1921 to "dumb down" the Hello message so that the old C3620 accepts it?

Thanks,

Richard Culpan - Artevea Digital Ltd.

[Peter Paluch · Accepted solution]

Hello Richard,

I do not know for sure but personally, I believe that the C3620 simply skips the unrecognized options and otherwise processes the PIM message.

However, you are talking about the registering process to the Auto-RP Send-RP-Announce group 224.0.1.39. Why would you want to do this? Cisco uses this group in Dense Mode operation and I would personally suggest experimenting with a different group that does not clash with special-use groups in Cisco devices like this one.

In any case, it would be very helpful to actually see whether the PIM Register messages sent from 1921 reach the 3620, and whether the PIM Register-Stop is ever generated and sent back from 3620 to 1921. These messages that constitute the PIM registering process are not related to the Hello messages and unrecognized options.

Best regards,

Peter

[richard-artevea]

Hi Peter,

Thanks for the reply.

You are right - after further troubleshooting and gettinmg the thing working (see below) I can see that the C3620 is just ignoring the PIM options it doesn't understand and not the entire message.

Being a newbie I had not appreciated that the register was carried in a different message...

As you suggested I traced the register message sent by the C1921; it _was_ getting lost (going to the sink RP instead of to the C3620). Culprit was a typo in the C3620 configuration (access list that allows the two groups 224.0.1.39 and .40 to work in dense mode if the RP is not reachable).

Offending access list deleted and reloaded - problem fixed. BTW, the extract from the 'show ip mroute' in my first post was for the attempoted join issued by the C1921 itself, not from any source PC I was mesing with...apologies for not explaining that.

Thanks,

Richard Culpan - Artevea Digital Ltd.