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:
1. | 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.
| 2. | 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).
| 3. | 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.
| 4. | 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.
| 5. | On the Host Parameters tab, input the IP and Subnet mask of the network adapter itself.
| 6. | 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.
| 7. | 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.
| 8. | Adding
the second CAS server to the mix is done using the Network Load
Balancing Manager, which is opened from Administrative Tools.
| 9. | 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.
| 10. | 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.
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If you want to perform the same actions in Server 2008, consider the following:
1. | In Server 2008, NLB is a feature you need to turn on in the Features. Open your Server Manager, and add the feature.
| 2. | 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.
| 3. | 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 0.0.0.0) 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.
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Note
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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