Multicast versus Unicast Modes

NLB clusters operate in either Multicast or Unicast mode. The default mode is Unicast. In this mode, the NLB cluster automatically reassigns the MAC address for each cluster member on the NIC that is enabled in cluster mode. If each member has only one NIC, member to member communications are not possible in this mode. This is one reason why it is best to install two NICs in each server.

When using the Multicast mode, NLB assigns two multicast addresses to the cluster adapter. This mode ensures that all cluster members can automatically communicate with each other because there are no changes to the original MAC addresses. There are disadvantages to this mode though, especially if you use Cisco routers. The address resolution protocol (ARP) response sent out by a cluster host is rejected by these routers. If you use Multicast mode in an NLB cluster with Cisco routers, you must manually reconfigure the routers with ARP entries mapping the cluster IP address to its MAC address.

Whether you use one mode or the other, you should use two NICs on each member. One advantage of doing so is that it allows you to configure one card to receive incoming traffic and the other to send outgoing traffic, making your cluster members even more responsive. You can also ensure that if your NLB cluster is only the front end of a complex clustering architecture such as the one illustrated in Figure 9-2, all back end communications are handled by the non-clustered NIC.

If your NLB members are expected to handle extremely high traffic loads, you can use Gigabyte Ethernet cards to improve communication speed and host only the essential networking services on each card (for example, Client for Microsoft Networks should definitely be turned off on clustered NICs). If even higher loads are expected, you can also add more NICs in each member and bind the NLB service to each one, improving the overall response time for each member.

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