Lesson 2

Using Network Load Balancing

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NLB server duties. There are also no problems with routers handling multicast MAC

addresses and no bottlenecks caused by cluster traffic and ordinary network traffic

sharing a single network interface adapter.

NLB Networking

Although the servers in a Network Load Balancing cluster do not share a single data

store, as in a server cluster, and perform their own independent calculations to deter-

mine which server will service an incoming request, the servers do communicate with

each other. The cluster servers must exchange information to know many servers are

in the cluster, and to determine when a server has been added or removed from the

cluster. This communication enables the cluster to compensate for a failed server and

to take advantage of new servers in the cluster by redistributing the traffic load.

Important

A single computer running Windows Server 2003 cannot be a member of a Net-

work Load Balancing cluster and a server cluster at the same time, because these two clustering

solutions use network interface adapters in different ways. If you want to deploy both an NLB

cluster and a server cluster on your network, you must use separate servers for each cluster.

The cluster traffic between NLB servers takes the form of a heartbeat message that each

server transmits once per second to the other servers in the cluster. If one cluster server

fails, it stops transmitting its heartbeat messages, and the other servers detect the

absence of the heartbeats. Once the other servers in the cluster miss five consecutive

heartbeat messages from a server, they begin a process called convergence, in which

they recalculate their traffic distribution algorithm to compensate for the missing server.

In the same way, adding a new server to an NLB cluster introduces a new heartbeat to

the network, which triggers a convergence in the other servers, enabling them to redis-

tribute the traffic so that the new server receives an equal share of the load.

Note

Because all the servers in the cluster are using the same cluster MAC address, trans-

mitting the heartbeats is simply a matter of directing the packets to that address. The serv-

ers don’t need to broadcast the heartbeat messages, reducing the impact of the cluster

traffic on the network.

When you deploy NLB cluster servers with a single network interface adapter in each

computer, obviously all the cluster-related traffic must travel over the same network as

your ordinary traffic. This is usually not a major burden, because the heartbeat packets

are small, less than 1,500 bytes, so they fit into a single Ethernet packet. If you decide

to install multiple network interface adapters in each cluster server, you can connect

both adapters to the same local area network (LAN) or construct a separate network for

the cluster traffic.