The NLB cluster uses a unicast IP address and non-IGMP multicast mac (03:bf) so IGMP is not used. N1k floods this frame.
This method could overwhelm the network in some situations.
1. Use a dedicated VLAN for NLB VMs to limit mcast replication & flooding.
NLB with Multicast+IGMP-
Microsoft violates RFC2236 by putting a unicast IP in the IGMP Group messages. N1k drops these messages since they violate the RFC. CSCue32210 - "Add support for Microsoft NLB - Multicast+IGMP mode in Nexus 1000v" is targeted for a future release. Before this feature exists we can configure the network as follows:
1. Dedicate a VLAN for NLB VMs to limit mcast replication & flooding.
2. Disable IGMP snooping on that vlan
no ip igmp snooping
3. Add a static entry on upstream router for NLB cluster IP & shared MAC.
int vlan 10
ip arp 220.127.116.11 0100.5e7f.7c28
4. Use mac-pinning configuration with manual pinning NLB vEths to one set of uplinks. This will isolate flooding to a single upstream fabric interconnect & switch.
port-profile type veth NLB-VM
channel-group auto mode on mac-pinning relative
pinning id 0 backup 1 <-these numbers may differ in your environment