Once you've configured QoS for your VoIP traffic, sometimes it's difficult to know if it's actually working or not. Your voice may sound good, but that doesn't necessarily mean your QoS is working for you. It could simply mean there isn't currently enough other traffic contending with your voice traffic for you to notice. Also, your voice traffic could be starving other traffic. Uncovering these problems can be difficult with the right tools, and next to impossible without.
Some of the right tools you'll need are a good protocol analyzer that has software that is QoS aware and can calculate your application performance in terms of latency and jitter, and track it over time. You'll want to insert the analyzer in the middle of a link using a tap. Do not use a span or mirroring port from a switch, as this will introduce some irrelevant and unpredictable distortion into your jitter statistics. The other tool you'll need is something to generate traffic in a consistent and controlled way. Use these tools together to make sure your delay and jitter are within reasonable limits.
You'll also want to keep an eye on a few statistics from inside your network devices. These will vary by manufacturer and product line, but the most useful statistics you'll want to track are
This is the number or size of packets in an interface's packet buffers. Your entire queuing scheme revolves around managing this buffer, so if you don't see the results you're expecting, you need to change your scheme.
Depending on your hardware's architecture, the CPU may or may not be a limiting factor.
If your queues fill up, dropping packets is your last resort. Which queue did you drop them from, and how often were they dropped? This may be a hint that other traffic is suffering too much.
These and other statistics are most frequently gathered using SNMP, but there are other ways to access them, such as Cisco's QoS Device Manager, which is a Java-based applet that runs on a web server built into IOS on a Cisco router.
Thomas Alexander Lancaster IV is a consultant and author with over ten years experience in the networking industry, focused on Internet infrastructure.