Understanding how QoS works is complicated enough, and configuring it is even worse. But knowing what values to put in is the real challenge. For instance, how big should your queue sizes be, and how many queues should you have and what classification should you use for different types of traffic and which types of traffic should you mark? How will your strategy differ on Gigabit backbone links versus the Fast Ethernet and WAN links? The list goes on and on.
Recently, Cisco created a feature, prominent on the 3550 series of switches, called Auto-QoS. As you might expect, you type in a quick command and the switch automatically sets up all the QoS you need to support an IP Telephony network. While that's pretty spiffy, it's not terribly useful for most people because it only works with Cisco's IP Phones via their proprietary Cisco Discovery Protocol (CDP). And as you know, not everyone uses Cisco's IP Telephony.
What IS cool about this though, is seeing the values that Auto-QoS selects as defaults. The following link has the gory details:
Regardless of what brand of phone or switches you're using, the numbers provided on this Web page are an excellent place to start if you don't have a lot of experience. Note that the regular commands are listed, so that you can use them on other platforms or if your version doesn't have Auto-QoS.
While these numbers may not be optimal for every situation, Cisco has obviously put some thought into them and chance are you'll only need some minor tweaking to get your QoS properly tuned.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.
This was first published in June 2003