When you set up a unicast Network Load Balancing (NLB) cluster, a large amount of broadcast network traffic will be generated on any switch to which a cluster node is connected. This is normal behavior for a unicast NLB cluster. You may not even notice this traffic unless you are running a packet capture from a machine connected to the same switch as the cluster nodes.
Normally, a switch builds a MAC address table by learning what ports a MAC address is communicating on. This automatic learning process only works if a given MAC address is unique across all the ports on a switch.
Because nodes in a unicast NLB cluster all share a common cluster MAC address, the network switch to which they are connected cannot learn which port the MAC address is tied to. Therefore it is never able to add the cluster MAC to its table. As a result, all traffic going to the cluster MAC is always broadcast out all switch ports.
This may or may not be a problem, depending on the amount of traffic going to your cluster and the amount of other traffic which is already being handled by the network switch. If it is a problem, there are several ways to resolve it.
1. Switch to a multicast or multicast IGMP NLB cluster. You will need to make sure your switches support multicast for this to work. Cisco switches with a relatively recent IOS should have this capability, but you should check first, to be sure.
2. Move the unicast NLB cluster nodes to a separate switch, where they are the only connected devices.
3. Set up a separate VLAN or network (dedicated router/firewall interface) just for the cluster, which will contain the broadcast traffic.
4. Add static MAC table entries on your switch to tell it which ports are being used by the cluster nodes. This way, traffic going to the cluster nodes would only be sent out the applicable ports. Each time you add another cluster node, you would also need to add an entry to the switch MAC table.
Option 4 is the easiest, and one that I have used in production on a small cluster.
All of these options will work; it’s really just your preference as to which one you use. As long as you document it, you’ll be in good shape in any case, right?
Here are some useful links regarding NLB: