Windows Server

Windows Server 2008 : Configuring Server Clusters (part 1)

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10/22/2010 5:59:22 PM

Server Cluster Fundamentals

In Windows Server 2008, you can configure three types of server groups for load balancing, scalability, and high availability. First, a round-robin distribution group is a set of computers that uses DNS to provide basic load balancing with minimal configuration requirements. Next, a Network Load Balancing (NLB) cluster (also called an NLB farm) is a group of servers used not only to provide load balancing but also to increase scalability. Finally, a failover cluster can be used to increase the availability of an application or service in the event of a server failure.

Note: What is load balancing?

Load balancing is a means of distributing incoming connection requests to two or more servers in a manner that is transparent to users. Load balancing can be implemented with hardware, software, or a combination of both.

Round-Robin Distribution

Round-robin DNS is a simple method for distributing a workload among multiple servers. In round-robin, a DNS server is configured with more than one record to resolve another server’s name to an IP address. When clients query the DNS server to resolve the name (find the address) of the other server, the DNS server responds by cycling through the records one at a time and by pointing each successive client to a different address and different machine.

For example, suppose that a DNS server authoritative for the DNS domain is configured with two separate resource records, each resolving the name by pointing to a different server, as shown in Figure 1. When the first client (Client1) queries the DNS server to resolve the, the DNS server answers the query with the information provided in the second record matching “web.” This second record points to a server name websrv2, which is located at the address. If a third client then queries the DNS server for the same name, the server will respond with information in the first record again. name, the DNS server answers by pointing the client to the server named websrv1 located at the address. This is the information associated with the first DNS record matching “web.” When the next client, Client2, queries the DNS server to resolve the same name (

Figure 1. Round-robin uses DNS to distribute the client load between two or more servers

The purpose of DNS round-robin is to load balance client requests among servers. Its main advantage is that it is very easy to configure. Round-robin DNS is enabled by default in most DNS servers, so to configure this simple sort of load balancing, you only need to create the appropriate DNS records on the DNS server.

However, there are serious limitations to round-robin as a load balancing mechanism. The biggest drawback is that if one of the target servers goes down, the DNS server does not respond to this event, and it will keep directing clients to the inactive server until a network administrator removes the DNS record from the DNS server. Another drawback is that every record is given equal weight, regardless of whether one target server is more powerful than another or a given server is already busy. A final drawback is that round-robin does not always function as expected. Because DNS clients cache query responses from servers, a DNS client by default will keep connecting to the same target server as long as the cached response stays active.

Network Load Balancing

An installable feature of Windows Server 2008, NLB transparently distributes client requests among servers in an NLB cluster by using virtual IP addresses and a shared name. From the perspective of the client, the NLB cluster appears to be a single server. NLB is a fully distributed solution in that it does not use a centralized dispatcher.

In a common scenario, NLB is used to create a Web farm—a group of computers working to support a Web site or set of Web sites. However, NLB can also be used to create a terminal server farm, a VPN server farm, or an ISA Server firewall cluster. Figure 2 shows a basic configuration of an NLB Web farm located behind an NLB firewall cluster.

Figure 2. Basic diagram for two connected NLB clusters

As a load balancing mechanism, NLB provides significant advantages over round-robin DNS. First of all, in contrast to round-robin DNS, NLB automatically detects servers that have been disconnected from the NLB cluster and then redistributes client requests to the remaining live hosts. This feature prevents clients from sending requests to the failed servers. Another difference between NLB and round-robin DNS is that in NLB, you have the option to specify a load percentage that each host will handle. Clients are then statistically distributed among hosts so that each server receives its percentage of incoming requests.

Beyond load balancing, NLB also supports scalability. As the demand for a network service such as a Web site grows, more servers can be added to the farm with only a minimal increase in administrative overhead.

Failover Clustering

A failover cluster is a group of two or more computers used to prevent downtime for selected applications and services. The clustered servers (called nodes) are connected by physical cables to each other and to shared disk storage. If one of the cluster nodes fails, another node begins to take over service for the lost node in a process known as failover. As a result of failover, users connecting to the server experience minimal disruption in service.

Servers in a failover cluster can function in a variety of roles, including the roles of file server, print server, mail server, or database server, and they can provide high availability for a variety of other services and applications.

In most cases, the failover cluster includes a shared storage unit that is physically connected to all the servers in the cluster, although any given volume in the storage is accessed by only one server at a time.

Figure 3 illustrates the process of failover in a basic, two-node failover cluster.

Figure 3. In a failover cluster, when one server fails, another takes over, using the same storage

In a failover cluster, storage volumes or LUNs that are exposed to the nodes in a cluster must not be exposed to other servers, including servers in another cluster. Figure 4 illustrates this concept by showing two two-node failover clusters dividing up storage on a SAN.

Figure 4. Each failover cluster must isolate storage from other servers

Configuring an NLB Cluster

Creating an NLB cluster is a relatively simple process. To begin, install Windows Server 2008 on two servers and then, on both servers, configure the service or application (such as IIS) that you want to provide to clients. Be sure to create identical configurations because you want the client experience to be identical regardless of which server users are connected to.

The next step in configuring an NLB cluster is to install the Network Load Balancing feature on all servers that you want to join the NLB cluster. For this step, simply open Server Manager, and then click Add Features. In the Add Features Wizard, select Network Load Balancing, click Next, and then follow the prompts to install.

The final step in creating an NLB cluster is to use Network Load Balancing Manager to configure the cluster. This procedure is outlined in the following section.

To create an NLB cluster

Launch Network Load Balancing Manager from Administrative Tools. (You can also open Network Load Balancing Manager by typing Nlbmgr.exe from a command prompt.)

In the Network Load Balancing Manager console tree, right-click Network Load Balancing Clusters, and then click New Cluster.

Connect to the host that is to be a part of the new cluster. In Host, enter the name of the host, and then click Connect.

Select the interface you want to use with the cluster, and then click Next. (The interface hosts the virtual IP address and receives the client traffic to load balance.)

On the Host Parameters page, select a value in the Priority (Unique host identifier) drop-down list. This parameter specifies a unique ID for each host. The host with the lowest numerical priority among the current members of the cluster handles all the cluster’s network traffic not covered by a port rule. You can override these priorities or provide load balancing for specific ranges of ports by specifying rules on the Port rules tab of the Network Load Balancing Properties dialog box.

On the Host Parameters page, verify that the dedicated IP address from the chosen interface is visible in the list. If not, use the Add button to add the address, and then click Next to continue.

On the Cluster IP Addresses page, click Add to enter the cluster IP address shared by every host in the cluster. NLB adds this IP address to the TCP/IP stack on the selected interface of all hosts chosen to be part of the cluster. Click Next to continue.

Note: Use only static addresses

NLB doesn’t support Dynamic Host Configuration Protocol (DHCP). NLB disables DHCP on each interface it configures, so the IP addresses must be static.

On the Cluster Parameters page, in the Cluster IP Configuration area, verify appropriate values for IP address and subnet mask, and then type a full Internet name (Fully Qualified Domain Name) for the cluster.

Note that for IPv6 addresses, a subnet mask is not needed. Note also that a full Internet name is not needed when using NLB with Terminal Services.

On the Cluster Parameters page, in the Cluster Operation Mode area, click Unicast to specify that a unicast media access control (MAC) address should be used for cluster operations. In unicast mode, the MAC address of the cluster is assigned to the network adapter of the computer, and the built-in MAC address of the network adapter is not used. It is recommended that you accept the unicast default settings. Click Next to continue.

On the Port Rules page, click Edit to modify the default port rules. Configure the rules as follows:

  • In the Port Range area, specify a range corresponding to the service you want to provide in the NLB cluster. For example, for Web services, type 80 to 80 so that the new rule applies only to HTTP traffic. For Terminal Services, type 3389 to 3389 so that the new rule applies only to RDP traffic.

  • In the Protocols area, select TCP or UDP, as needed, as the specific TCP/IP protocol the port rule should cover. Only the network traffic for the specified protocol is affected by the rule. Traffic not affected by the port rule is handled by the default host.

  • In the Filtering mode area, select Multiple Host if you want multiple hosts in the cluster to handle network traffic for the port rule. Choose Single Host if you want a single host to handle the network traffic for the port rule.

  • In Affinity (which applies only for the Multiple host filtering mode), select None if you want multiple connections from the same client IP address to be handled by different cluster hosts (no client affinity). Leave the Single option if you want NLB to direct multiple requests from the same client IP address to the same cluster host. Select Network if you want NLB to direct multiple requests from the local subnet to the same cluster host.

After you add the port rule, click Finish to create the cluster.

To add more hosts to the cluster, right-click the new cluster, and then click Add Host To Cluster. Configure the host parameters (including host priority and dedicated IP addresses) for the additional hosts by following the same instructions that you used to configure the initial host. Because you are adding hosts to an already configured cluster, all the cluster-wide parameters remain the same.
Other -----------------
- Windows Server 2008 : Configuring Server Storage (part 3) - Configuring a Mount Point
- Windows Server 2008 : Configuring Server Storage (part 2) - Managing Disks, Volumes, and Partitions
- Windows Server 2008 : Configuring Server Storage (part 1)
- Use the Microsoft Management Console (MMC)
- Manage Windows Server 2008 : Work with Preconfigured MMCs
- Manage Windows Server 2008 : Work with the Task Scheduler
- Manage Windows Server 2008 Using Remote Desktop
- Manage Windows Server 2008: Configure Backups and Perform Restores
- Windows Server 2008 : Determine Which Terminal Services Roles to Install
- Windows Server 2008 : Install the TS Gateway Role Service and TS Web Access Role Service
- Windows Server 2008 : Install the TS Licensing Role Service
- Windows Server 2008 : Install the Terminal Server Role Service
- Windows Server 2008 : Configure a Load-Balanced Farm with TS Session Broker
- Windows Server 2008 : Configure the TS Gateway Manager
- Windows Server 2008 : Configure the TS RemoteApp Manager
- Windows Server 2008 : Manage Terminal Services
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