Design and Deploy High Availability for Exchange 2007 : Design CAS High Availability

10/12/2010 3:53:19 PM
Installing additional Client Access servers provides multiple points for access, but not necessarily high availability. Remember, your CAS servers are responsible for OWA, ActiveSync, and several other solutions that require availability through well placed and uniquely configured CAS servers.

However, keep in mind that the majority of the services provided through a CAS server are handled through your IIS settings. Network Load Balancing (NLB) clusters provide high availability for IIS.

In the event you want higher availability for a CAS server, you can always use NLB. On the one hand, NLB distributes client requests across multiple servers, so that offsets some of the load. On the other hand, if one server goes down, the others continue. NLB is also provided for both the Standard and Enterprise versions of Server so you don’t need any special software to implement it.

Logically, there are some differences in how to configure NLB, depending on whether you are using Server 2003 or Server 2008. To enable NLB on your CAS server (running 2003), perform the following:

Create a record for the NLB cluster in your DNS Manager. To do this, click Start, Administrative Tools, and then DNS. Expand the Forward Lookup Zones container, right-click the zone for your AD, and click New Host (A). Type the name and IP address (within the same subnet as your two CAS servers) you want to use for the NLB cluster.

Your next step is to configure your network adapters. Each CAS server should have two adapters for the NLB cluster. One is Public and the other Private (or NLB). First, you might want to name the connections, so that you can easily locate the right one when you return to the servers. Then change the binding order so the Public connection comes first (which is done by going into Advanced Settings from the Network Connections dialog and under the Adapters and Bindings tab, making sure the Public connection is first).

To enable NLB on the first server, go to your NLB adapter and go into the properties. On the General tab, you can turn on Network Load Balancing and go into the properties.

Under Cluster Parameters, enter the IP address and subnet mask as well as the full internet name. These are the same settings we configured in DNS. Make sure the cluster operation mode is Unicast.

On the Host Parameters tab, input the IP and Subnet mask of the network adapter itself.

On the Port Rules tab (delete the default), configure the port range for those connections that you have coming in (HTTP, SSL, POP, IMAP) depending on the work of our CAS servers.

After you have the settings complete, you need to add the cluster IP address to the TCP/IP property page of the network adapter itself. To do this, go into the Advanced TCP/IP Settings off the adapter and add the IP address.

Adding the second CAS server to the mix is done using the Network Load Balancing Manager, which is opened from Administrative Tools.

To add the second server, select Cluster from the menu and then Add Host. Type the name of the second CAS server, and then click Connect. Select the cluster name and click Finish.

You are taken to the Host Parameters tab. Type the IP address and subnet mask of the adapter that is part of the NLB cluster. Then you see in the manager that the two servers are NLB-connected.

If you want to perform the same actions in Server 2008, consider the following:

In Server 2008, NLB is a feature you need to turn on in the Features. Open your Server Manager, and add the feature.

With Server 2008, you can go through your Network and Sharing Center and turn NLB on for the adapter you want to enable NLB clustering on; however, to configure it, you are told to go to the Network Load Balancing Manager, which is found under Administrative Tools.

After you open the Network Load Balancing Manager (after you have turned on NLB for your adapter), notice that you have the initial part set up, but it is Unconfigured in the Status. To fix this, select your connection (which might be and go to the Cluster Properties. Here, you see familiar screens if you have done this before in Server 2003—Cluster IP Addresses, Cluster Parameters (shown in Figure 1), and Port Rules.

Figure 1. Configuring cluster parameters.


In the next section, we discuss the capability of the Hub Transport server role to automatically load balance when you have more than one in an Active Directory site. However, if you need to load balance (which is necessary under certain situations), configuration is the same as what we have here for the CAS server.

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