Network Load Balancing Clusters - Page 2

 By Brien M. Posey
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Each Server Contains Multiple Network Adapters Running in Unicast Mode

Configuring your clustered servers to run in unicast mode with multiple network cards is considered by many to be the preferred configuration. Sure, this configuration is a little more expensive than single card models, but network cards are cheap these days. Even the extra hub ports that will be required are much cheaper than they were a year or two ago.

This particular NLB model offers excellent network performance, because all load balancing traffic is sent through a dedicated backbone that connects to one network card on each server. Because this backbone isn't connected to the general network in any way, traffic can flow freely without becoming congested by the usual network traffic. Likewise, your clients won't notice a slow down because communications between servers don't place additional traffic on the line that the clients use.

As you might have guessed, this model doesn't impose any limitations as to ordinary communications between the servers. NetBIOS support is available across one adapter on each server, and this NLB model works with all routers

Each Server Contains Multiple Network Adapters Running in Multicast Mode

You'd use this model only if some of your servers contain multiple network adapters and some have only a single network card. If this is the case, you're forced to use multicast mode because you can't mix unicast and multicast in an NLB environment. This particular solution may cause problems with communication across some types of routers; but assuming that your router is up to the job, this solution works well until you can add a second network card to the servers that contain only a single network card.

Capacity Planning

Before I conclude this article, I'd like to say a few words about capacity planning in an NLB environment. As I've mentioned, an NLB cluster may have between 2 and 32 servers. Generally speaking, a few really fast servers will perform better and cost less than several slower servers. Although 32 servers is the limit, there is a way of using more. You can use several different clusters, each containing up to 32 servers. You can then use your DNS server to rotate the clusters that it assigns to clients in round-robin style. //

Brien M. Posey is an MCSE who works as a freelance writer. His past experience includes working as the director of information Systems for a national chain of health care facilities and as a network engineer for the Department of Defense. Because of the extremely high volume of e-mail that Brien receives, it's impossible for him to respond to every message, although he does read them all.

This article was originally published on Oct 7, 2000
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