Using teaming adapters with network load balancing may cause network problems

Some hardware manufacturers offer fault tolerance with their network adapters and accompanying drivers. Fault tolerance technology lets you group network adapter ports for a connection to a single physical segment. If connectivity through one port does not work, another port is opened automatically. This operation is transparent to the operating system and other devices on the network.

Note Support for fault-tolerant technology is the responsibility of the hardware manufacturer. (Fault-tolerant technology includes the hardware and the drivers.)

When you use Network Load Balancing (NLB), network adapters that provide fault tolerance may cause problems. When you use NLB in unicast mode, NLB uses a shared virtual media access control address. The shared virtual media access control address is used instead of the physical adapter's media access control address for all communications to the load-balanced IP addresses. With some teaming adapters, the virtual media access control address cannot to overwrite the physical media access control address. This causes in IP address conflicts in the cluster.

Note If you use Application Center 2000 to configure NLB, it will set NLB to unicast mode.

If you use NLB in unicast mode, you may have to manually set the card's Locally Administered address (LAA) to the NLB cluster unicast MAC address. To do this, you must configure the driver software for the load balanced adapter on each computer in the cluster. Do not set the LAA for NLB clusters that operate in multicast mode.

Important If you manually set the unicast LAA, you must also manually remove the unicast LAA when you remove the server from the cluster. If you do not remove the unicast LAA when the server is removed from the cluster, you may receive MAC address conflicts.

This difficulty occurs only on the NLB interface and does not affect teamed, non-clustered interfaces on the same server. Many hardware manufacturers have updated drivers to correct this problem. Also, when you use multicast instead of unicast for NLB, NLB can function in a network adapter teaming environment because NLB does not overwrite the physical media access control address. From the perspective of Microsoft Product Support Services (PSS), use of teaming on clustered or dedicated interfaces is acceptable. However, if problems that occur seem to be related to teaming, PSS may require that you disable teaming while the problem is investigated. If this disabling of teaming itself resolves the problem, you must seek assistance from the hardware manufacturer.