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Simon O'Sullivan

Should unknown multicast traffic be flooded if IGMP snooping is enabled?

Hi all,

We have recently upgraded some switches from Catalyst 4506 to Catalyst 6807 Sup6T.

Since moving to the 6807, we have found that some multicast traffic is no longer delivered around the network as expected.

The traffic is L2 only - no routing involved.

Can anyone tell me what is the expected behavior of multicast traffic when:

IGMP snooping is enabled

the hosts participating in the multicast traffic do not send IGMP messages

Should the traffic be flooded to all ports in the vlan (as unknown multicast)?

What settings should I look for to ensure that the (unknown) multicast is flooded?

What we are seeing is that the 4506 floods the multicast traffic, but the 6807 does not.

The 6807 does see that it is receiving multicast traffic on a port because the interface counters for multicast increment.

We have tested this with two deivces connected directly to ports on the 6807 (no other switches involved).

Thanks, Simon.

Rising star

Hi Simon

Take a look at this document, it explains it quite well.



Hi Mikael,

Thanks I saw that doc previously, but it doesn't help. I have found more detail on the issue though...

What happens is that lots of devices do make use of MultiCast traffic but do not make use of IGMP and do not send IGMP report messages. An example of such devices are devices that make use of "Precision Time Protocol" (PTPv2).

On most Cisco Catalyst switches with IGMP snooping enabled:

When it sees multicast traffic for a destination that it has not seen any IGMP reports for it will flood that traffic out all ports in the vlan (normal switch behavor for unknown destination mac address).

On a Cisco 6807 with IGMP snooping enabled:

when it sees multicast traffic for a destination that it has not seen any IGMP reports for it will drop that traffic.

Unfortunately according to Cisco the only fix on a 6807 is to disable IGMP snooping on the entire vlan (or setup static MAC entries which is not scalable). Configuring an IGMP querier does not help as the devices in question do not do IGMP.

Thanks, Simon