Microsoft Network Load Balancing–Not Working Juniper Switching EX-4200

I recently ran into a problem where MS NLB would converge the 2 node web cluster but would not respond to packets.  the two nodes were VM’s on a 2008 R2 Hyper-V Cluster.  If you moved the nodes to the same Parent Partition they would respond to traffic if the traffic was generated on the same virtual switch.

Went through the normal troubleshooting to make sure that MAC spoofing was enabled on the vNICs etc.

New to the environment was a Juniper EX-4200 Virtual Chassis the switch has IGMP-snooping enabled by default.

the NLB was setup to use a single vNIC with Multicast and IGMP, however the juniper switch was required to disable IGMP-snooping in order to allow unicast traffic to the multicast address.

The NLB IP address on the servers is assigned a Multicast address however the MAC is used for unicast traffic, the juniper switch was blocking the MAC address from entering the Layer2 forwarding table as it assumed it was a multicast packet form the MAC address.

Once we disabled IGMP-snooping on the transit switch the NLB IP was allowed to pass through the Juniper switch, however the confusing part is the MAC address never showed up in any of the forwarding tables of either the Cisco or Juniper Switches, how ever it did pass the traffic.

to disable IGMP-snooping on the EX switches use this command

set protocols igmp-snooping vlan <vLAN Name> disable

commit sync and-quit

the protocol config should look like this

protocols {
    igmp-snooping {
        vlan vlan-name {


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1 Response to Microsoft Network Load Balancing–Not Working Juniper Switching EX-4200

  1. Hi

    Thanks for sharing good stuff

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