There was a discussion last week on one of the forums that I participate in on load testing WANs that got me to pondering the subject so I thought I'd write about it a little. If you've never been tasked with a load testing or WAN performance benchmarking project you're really missing out. These can be fun projects and in my experience offer some of the best learning experiences out there. Additionally, this is a vital part of any network assessment or application readiness project. So, even if you haven't been asked to do one - maybe it's time.

As you prepare to dive into the world of load testing there are some key things to keep in mind. First, let's deal with some of the common mistakes that people make. One common misconceptions about network performance is that bandwidth utilization equals network performance. While understanding and monitoring bandwidth utilization is a key part of analyzing network performance, it's really only one piece of the puzzle and depending upon the nature of traffic on the network it may not be the key piece. Latency, on the other hand, is often overlooked but is definitely a key metric to track in any performance analysis. Another common mistake is that people tend to think about both bandwidth consumption and latency in terms of single measurements, but it's important that you understand and measure each direction that the traffic is flowing as most WAN circuits are full-duplex, many are asymmetrical, and latency measurements can vary greatly depending on which direction that the traffic is going.

When it comes to inducing congestion (i.e. generating traffic) there are many ways to do this. Typically I use the WAN Killer application within the Engineer's Toolset here at SolarWinds. There are also some great open source traffic generators like Seagull. Of course, if you're in the mood to spend some money and wanna get really giggy with it there are a host of applicance based solutions on the market that you can go completely crazy with.

When using a traffic generator, like say for instance the WAN Killer, if you want to see traffic in both directions you'll usually need to enable simple TCP services (and or UDP services) on the target host. On a Windows server this involves opening the control panel and enabling Simple TCP Services. Within Cisco IOS the commands are:

service tcp-small-servers
service udp-small-servers

This is a subject that I could talk about forever, but I've found that blog entries tend to lose their appeal if they get too long so I'll wrap it up for now. Comment back if you want more info or feel free to ping me directly.

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