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.
