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MS NLB in IGMP multicast mode with IGMP Snooping

We have a NLB cluster operating in IGMP multicast mode, but we still see traffic for the cluster VIP flooded out all the switch ports. Looking at Cisco documentation it seems like if IGMP snooping is enabled and the cluster is operating in IGMP multicast mode, that the traffic should be limited to only the ports that have joined that multicast group, just like any other multicast traffic.

I can verify that IGMP snooping is turned on and the group & ports look correct and the group is registered via IGMP on the Netscreen that is acting as the gateway for the network.

Any thoughts on what else we can do so that switches only send the traffic to ports attached to the cluster members? We really don't want to create static MAC table entries since we would have to do this on every switch which contained that VLAN.

The cluster's unicast address is and it has two members, one connected to Gi3/0/6 on a 3750 and one connected to Gi10/4 on a 4510. Our gateway is connected to port Gi1/0/35 on the 3750 and the two switches are connected via Te1/0/1.

3750#show ip igmp snoop groups vlan 100

Vlan Group Type Version Port List


100 igmp v1 Gi1/0/35, Te1/0/1,


4510#show ip igmp snoop groups vlan 100

Vlan Group Version Port List


100 v1 Gi10/4

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Re: MS NLB in IGMP multicast mode with IGMP Snooping

Hello Joe,

IGMP snooping works correctly when there is a L3 multicast device that performes periodic IGMP queries that is the netscreen in your case.

The show commands look like well so it is difficult to say what is going wrong.

Usually the issue with IGMP snooping turned on is the opposite : that if an IGMP querier is missing all multicast traffic is blocked.

In your case it looks like as not effective.

Do you see the multicast traffic on unwanted ports of vlan 100 on both switches or only on one ?

Another possible issue could be the IGMP version I see version 1 detected in the show commands (but it shouldn't be a problem).

Hope to help