Implementing High Availability for Outlook Web Access using Network Load Balancing

The Concept

The idea of NLB is to provide a full high available service without any single point of failure. Other load balancing features need a dispatcher that collects all incoming requests and forwards them to one of the nodes. NLB does not need a dispatcher, it is fully software implemented and runs like a driver on the network card interface called Network Load Balancing. By default this feature is installed but not activated. Before activating this feature you will need to know how it works.

NLB works with all socket based applications and supports up to 32 nodes. It automatically provides a virtual MAC address for the virtual server running on all nodes. In addition it will provide the same IP Address for all nodes.

The algorithm running on NLB services provides automatism that a started node is included in the cluster and a powered-down node is excluded within 4-10 seconds.

The configuration is quite easy due to the NLB Configuration Snap-in provided with Windows Server 2003. Due to general understanding problems of NLB concepts we will have a hard look at the manual configuration itself. If you want to, you can configure the same settings using the snap-in wizard.

Configuring Network Load Balancing

Figure 1:
Activation of Network Load Balacing Services

Figure 2: Configuring the Cluster Node Parameters

The “virtual” Cluster Node IP Address is the reserved IP for the cluster on which you can connect to in the future. You can use two modes: Unicast MAC or multicast MAC

In the unicast mode NLB disables the onboard MAC address for the cluster adapter. The dedicated IP address cannot be used for interhost communications because all of the hosts have the same MAC address. In multicast mode NLB supports both the onboard and the multicast address. If your application will require communications using NetBIOS, use multicast mode or install a second network adapter.

Figure 3: Configuring Host Parameters

In the second register card of NLB properties you have to configure the dedicated IP address. And in addition you have to define the unique priority identifier of the node.

Figure 4: Configuring Port Rules for NLB

Figure 5: Editing port rules

Port rules are very important within your NLB configuration. At first you can define the port range on which NLB runs, in addition you can configure the filtering modes: the way your cluster behaves. The default setting is that the load is spread on each node in equal time.

Preparing OWA for Network Load Balancing

After having configured NLB properly on all nodes you can now configure your Exchange Server 2003 Front-end Servers. In Exchange System Manager go to your server object, in properties configure it as Front-end Server. The only thing you need to configure to make them run on NLB services is to configure the virtual IP address of the cluster.

Figure 6: Using NLB on Exchange Server 2003 Front-end Sservers

Now all your users are able to use your NLB clustered Exchange Front-end Server farm using:



With Exchange Server 2003 and the Network Load Balancing feature of Windows Server 2003 you can easily configure a high available Oultook Web Access infrastructure without investments in additional hardware components for your network.

It can be easily monitored, too. For example there is a management packet for Microsoft Operations Manager 2005 and the NLB feature is fully WMI scriptable.

Further information can be found at:

If there are still questions, please do not hesitate to contact me.

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