The New WLBS

In "Load Balancing Down on the Web Farm," February 2002, InstantDoc ID 23572, I explored the ins and outs of Windows NT Load Balancing Service (WLBS) 2.0, which adds fault tolerance for IIS 4.0 installations. WLBS's successor—Network Load Balancing (NLB)—is currently available only with IIS 5.0 on Windows 2000 Advanced Server or Win2K Datacenter Server. However, Microsoft will also offer NLB with the new "locked-down-for-security" IIS 6.0. I show you how NLB compares with WLBS; then, I show you how to configure both NLB and the TCP/IP stack.

NLB and WLBS
NLB has several improvements over WLBS, most noticeably in the service setup. The software is installed with the OS, so you don't have headaches with network bindings. To get NLB up and running, you simply select the service in the TCP/IP properties; supply the proper addresses, port rules, and priorities; then tune the server's TCP/IP stack for NLB.

If NLB's improved setup runs circles around WLBS, WLBS continues to lead in cost. Table 1 shows some of the initial WLBS and NLB deployment costs. The table shows three OS options—NT 4.0, Enterprise Edition (NTS/E), Win2K Server with Microsoft Application Center 2000 installed, and Win2K AS. All the prices you see in the table are street prices on a per-server basis. You might pay less if you have an existing Microsoft Select, Volume, Open, or Academic agreement.

So, which option should you choose? If you're looking for additional features that Application Center provides, such as COM component object instantiation load balancing and content deployment, then the equivalent pricing steers you toward that OS option. If you don't need those features and you'd rather have the additional processors and memory available to the Web server, consider the Win2K AS option. Similarly, if you already have a server running Win2K and you'd just like to add Application Center to the mix, the Win2K AS option might be better. (Although last month I strongly suggested that you run only like software and like hardware between cluster hosts for maximum uptime, Microsoft contends that you can have a mix of WLBS and NLB servers in the same cluster. The examples I use here demonstrate that the version of NLB included with Windows .NET Enterprise Server is compatible with the version of NLB that Application Center provides.) At the time of this writing, the latest version of NLB was in .NET Enterprise Server, build 3505.

Here's one more purchase factor to consider: If you're going to authenticate Web users, Win2K requires an Internet Connector License. This requirement is likely to continue when .NET Enterprise Server appears. This license costs about $1500 beyond the prices you see in Table 1. The NT 4.0 option doesn't require such a license. (For more information about Internet Connector Licenses, see the Microsoft article "Internet Connector License Types for Windows 2000" at http://support.microsoft.com/default.aspx?scid=kb;en-us;q272235.)

NLB Configuration Options
NLB supports several different configurations, such as support for one or more network adapters. The service can also support 2 to 32 hosts in the cluster, each with multiple NICs. (For more information about NLB, see Tim Huckaby's Windows 2000 Magazine article "The Tao of Network Load Balancing," September 2001, InstantDoc ID 21838.) To decide which configuration option is right for you, ask yourself this question: Do you expect heavy network traffic on the virtual IP interface? By heavy, I mean that the expected NIC traffic on the virtual IP (VIP) adapter would be heavy enough to potentially interfere with usual cluster heartbeat communications. (For more information about cluster heartbeat communications, see "Load Balancing Down on the Web Farm.") You might also have some security requirements that prohibit you from having cluster communications on the same interface. If the answer to the question is yes, think about setting up NLB with two NICs rather than one. If the answer is no, you should be completely satisfied with a single-NIC installation of NLBS. Last month, I showed you how to configure WLBS for dual NICs; this time, I show you the single-NIC installation of NLB. For information about dual-NIC NLB installation, see the Web-exclusive sidebar "Installing NLB on a Dual-NIC System," which is available at http://www.windowswebsolutions.com, InstantDoc ID 23885.

In addition to deciding how many NICs to use, you also need to decide which load-balancing mode to use. Like WLBS, NLB has two modes—Unicast and Multicast. Deciding between these modes sounds rather trivial, but you need to choose the best option for your network. In single-NIC installations, the default mode—Unicast—uses the derived media access control (MAC) address for all cluster-host and dedicated-host communications. The Multicast mode uses both the derived MAC address and the dedicated MAC address on the same NIC. Not all routers and switches support the Multicast feature. Likewise, network adapters that use teaming also have problems with the Multicast mode.

If the cluster hosts need to be able to communicate among themselves or you expect cluster traffic to be high, you might need multiple NICs. The default Unicast mode in multiple-NIC configurations is compatible with most routers and lets you separate the cluster traffic from the dedicated cluster-host traffic. Keep in mind that all hosts in an NLB cluster see the same traffic regardless of whether you have 2 hosts or 32 hosts in the cluster. An algorithm that every server in the cluster runs helps decide which server responds to the client request.

Setting Up NLB
Now that you know the NLB options and versions, let's install the service. Win2K AS and Datacenter have almost identical screens and installation instructions. I performed my setup on a .NET Enterprise Server machine. You'll be able to configure and change the private and virtual interfaces without having to reboot your servers. To demonstrate the heterogeneous design of WLBS and NLB, the second machine in my cluster runs Win2K Server Service Pack 2 (SP2) and Application Center. Figure 1 shows this cluster's setup.