The Tao of Network Load Balancing

The Single host mode. The Single host mode redirects network traffic intended for an associated range of ports to a specific host in the NLB cluster. It works in conjunction with the Handling priority parameter to determine which host handles the ports' traffic in the event of a failure.

The Disabled mode. The Disabled mode blocks all network traffic on a range of ports. You can use this mode to build a basic firewall (most likely supplemental to a full-featured firewall) to prevent network access to specific ports.

On the Local Area Connection Properties page, you must configure TCP/IP for NLB. The following description configures NLB on one NIC, but configuration on multiple NICs is straightforward. Click Internet Protocol (TCP/IP), then click Properties. In the IP Address field, type the address that you entered as the Dedicated IP address on the Network Load Balancing Properties dialog box's Host Parameters tab. This address is probably already set because it's the machine's static IP address. When you press the Tab key, the subnet mask will resolve automatically. Click Advanced, then click Add. Enter the NLB cluster IP address in the IP address field. When you press Tab, the subnet mask will resolve automatically.

Putting NLB into Action
To help you test your load-balancing capability, I've written a simple Active Server Pages (ASP) file that introduces load on a server by iterating the ASP request object's SERVERVARIABLES collection. The SERVERVARIABLES collection is a list of configuration parameters and settings specific to the Web server upon which you iterate it. For example, on an IIS 5.0 Web server, SERVER_SOFTWARE returns "Microsoft-IIS/5.0." (You can download the nlbtest.asp file from the Windows 2000 Magazine Web site at http://www.win2000mag.com. Enter InstantDoc ID 21838.) The ASP file also displays the name of the server on which you execute it.

I've propagated the nlbtest.asp file to each of the three hosts in my cluster. To do so in your environment, go to a machine—preferably one outside the cluster. Start multiple instances of a Web browser and in each one, navigate to the nlbtest.asp page at the NLB cluster's IP address. In my case, the page's location is http://192.168.1.100/nlbtest.asp. NLB will send each subsequent browser request to a different host in the cluster, thereby balancing the load. You can confirm that NLB has load-balanced each request because NLB will output to each browser the specific name of each server on which it has run. As Figure 5 shows, the server name is different in all three browser instances.

Only the Beginning
This article barely scratches the surface of NLB's configurations and features. NLB doesn't only provide scalability and high availability to TCP/IP protocol services such as IIS. Enterprisewide TCP/IP services such as Lightweight Directory Access Protocol (LDAP), Win2K Server Terminal Services, proxy servers, VPNs, and streaming media services also benefit greatly from NLB. One of my enterprise clients even uses NLB to distribute the printing load among Win2K servers dedicated to hosting printers.