IGMP and active ports

[Kelvin Willacey]

I have a pair of 9K switches and I needed to disable IGMP snooping on a number of VLANs. Before I removed the configuration I took a look at the 'show ip igmp snooping' output and for each VLAN it listed a set of active ports. After disabling IGMP snooping the active ports were still being listed in the output:

IGMP Snooping information for vlan 3
IGMP snooping disabled
Lookup mode: IP
Optimised Multicast Flood (OMF) disabled
IGMP querier none
Switch-querier disabled
IGMPv3 Explicit tracking enabled
IGMPv2 Fast leave disabled
IGMPv1/v2 Report suppression enabled
IGMPv3 Report suppression disabled
Link Local Groups suppression enabled
Router port detection using PIM Hellos, IGMP Queries
Number of router-ports: 0
Number of groups: 0
VLAN vPC function disabled
Active ports:
Eth1/10 Eth1/20 Po2

TAC has indicated that this should be ignored and the active ports only show up because the multicast traffic is being flooded since IGMP is disabled. I assumed this output would only be relevant to IGMP. Anyone with any experience can shed some light on this, thanks.

[paul driver]

Hello 

Igmp snooping assits it controlling the flooding of multicast traffic to specific hosts that request it to join an MC group, However once you have disabled snooping, As you have been informed by TAC, IGMP will flood MC to all hosts on the vlan/switch which require it or not. this will include also any MC routers that are connected

res

Paul

[Kelvin Willacey]

Thanks Paul. So is it that the ports that are listed as active are not directly related to IGMP? Rather they are considered active simply because there is multicast traffic?

[Jon Marshall]

Haven't used those switches but easiest way to tell I would have thought is if those ports are the complete list of ports for the vlan where the multicast traffic is.

In other words because the multicast traffic is now being flooded within the vlan then all ports for that vlan should show as active and if they don't then the active ports output is telling you something else.

Jon

[Kelvin Willacey]

Thanks for the input Jon. It appears as if the active ports match up with the complete list of ports that are in the respective VLANs.

[InayathUlla Sharieff]

HI, If you disable the igmp snooping now the traffic of multicast would be treated as broadcast for that particular vlan. Irrespective of receivers interested you would see traffic flowing across to all the ports of that vlan.

Regards

Inayath