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Windows Server NLB VIP Multicast Mode Through Cisco Switch

If you are unable to connect to a Windows Server Network Load Balancing (NLB) Virtual IP address configured for Multicast Mode it is because of the way your Cisco switch interacts with Microsoft NLB.

Symptoms

  • Cannot connect to Windows Server NLB Virtual IP address
  • Cannot ping Windows Server NLB Virtual IP address
  • Microsoft NLB Multicast Mode not working

I won’t go into the details of how NLB works but in short, client requests are distributed across different servers. To configure NLB, you can use three modes:

  • Unicast
  • Multicast
  • IGMP

Using Multicast Mode

When using Multicast mode, the cluster members will respond to ARPs using their virtual IP address and a multicast MAC address. Without the proper configuration on the switch you will not be able to connect to the Virtual IP address.

This will not work because within the ARP request packet is a unicast IP address and a multicast MAC address. Troubleshooting it can be frustrating but a Cisco switch will ignore this. Below is the configuration to be done on the switch.

Cisco Switch Configuration

To resolve the problem, create a static ARP entry for the Virtual IP Address to the NLB MAC address. Then create a static MAC address entry to the VLAN and interfaces used by the cluster.

In my case, my virtual IP address is 192.168.10.10 with MAC address of 03bf.0a65.05fa. That IP address is on VLAN 10 and the servers’ interfaces are part of a port-channel, both Port-Channel1 and Port-Channel2.

The following configuration is as follows:

arp 192.168.10.10 03bf.0a65.05fa
mac address-table static 03bf.0a65.05fa vlan 10 interface Port-Channel1 Port-Channel2

Some configuration items to note.. not all Cisco switches will support a static entry of a MAC address to multiple interfaces. In my case I am using a Cisco Catalyst 4500-X in VSS mode. The servers had two network interfaces configured into an EtherChannel.

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5 thoughts on “Windows Server NLB VIP Multicast Mode Through Cisco Switch”

  1. Keith Miller says:

    Ran into this a couple of years ago when migrating from some 6500s/3750s to Nexus 7Ks, 5Ks and 2Ks. Was my first initiation to Microsoft NLB and a pain to try to troubleshoot in the middle of a migration. Good post!

    1. Hey Keith! Thanks for chiming in 🙂

      I ran into this again just a couple of days ago and had to reference my own post. Sometimes I post them on my blog so I can remember. Thanks for stopping by!

      1. Keith Miller says:

        No worries Rowell, was a great post.

        I’m starting to think the same thing. I’ve learned so much over the past couple of years, it’s hard to remember it all.

  2. maione says:

    Where did you find the MAC address of the VIP? I don’t see it listed anywhere.

    1. Hi maione,

      I found the VIP from the server using Windows NLB. There’s usually an interface that will display the VIP and the MAC address from the server side.

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