Cisco AutoQoS: VoIP QoS for Mere Mortals

By Charlie Schluting | Sep 10, 2008 | Print this Page

Charlie SchlutingWANs often need Quality of Service (QoS) configured to ensure that certain traffic is classified as “more important” than other traffic. Until now, it took a serious Cisco guru to configure a network properly for VoIP if the network was at all bandwidth constrained. AutoQoS, a new IOS feature for Cisco routers, makes deploying VoIP easy, even on busy WAN links. In this article we’ll cover the basics, what AutoQoS does, and some of its limitations.

With its first whack at AutoQoS, Cisco recognized the need to simplify VoIP traffic prioritization. VoIP is especially sensitive to any latency, jitter, or loss, and users will notice problems. To ensure the best possible VoIP call, the network must ensure that lower priority traffic does not interfere with time-sensitive VoIP. AutoQoS can be enabled on both WAN links and Ethernet switches to automatically provide a nice best-practices based template for VoIP prioritization.

How It Works

QoS allows a router to classify which types of traffic are most important, and ensure that that traffic passed as quickly as possible. If necessary, other traffic will be queued until the higher priority traffic has had a chance to pass. Before a router can know when to queue versus when to attempt to pass all traffic, it must be configured with bandwidth settings for each link.

Configuring QoS on a Cisco router normally involves a complex series of interactions, which require understanding not only the protocols, but a router’s strange way of associating policies. The basic steps are:

  • Use an ACL to define which traffic gets matched
  • A class-map classifies matched traffic into classes
  • A policy-map assigns priorities to the classes
  • The policy-map is applied to the interface, which enables the processing of all packets through the ACL, class-map, and policy-map

Each of these “maps” are quite complicated and prone to error. Most sites are going to be duplicating effort because of common problems, like VoIP, needing QoS help.

Why AutoQoS?

QoS configuration is not simple. It requires understanding the protocols your network interfaces are using, as well as the type of data you’re passing. To configure QoS for VoIP, for example, you must understand how VoIP works. In short, it requires a guru. If you’re like me, you literally giggled out loud the first time you encountered the word, “AutoQoS.”

AutoQoS enables any network administrator to just “turn on” a solid solution for ensuring VoIP is happy. VoIP is the pain point for most organizations, so that’s what Cisco focused on first, and that’s what we’re focusing on here. Given the limited scope of AutoQoS, it’s believable that it works well enough. In reality, QoS configurations generally classify many types of traffic, and then place a priority on each one.

The main benefit of AutoQoS is that administrator training is much quicker. It also means that VoIP deployments often go much smoother, and upgrading WAN links isn’t usually required. Finally, AutoQoS creates templates that can be modified as needed and copied elsewhere for deployment.


Before talking about how to enable AutoQoS, which is literally three commands, let’s talk about where this works best, and what’s required to use AutoQoS.

First and foremost, you can only configure AutoQoS on a few types of router interfaces. These interfaces include:

Cisco Catalyst switches also support an AutoQoS command to prioritize Cisco VoIP phones, but you cannot prioritize generic VoIP protocols with it.

Next, there are some limitations with ATM sub-interfaces. If you have a low-speed ATM link (less than 768Kbps), then AutoQoS will only work on point-to-point sub-interfaces. Higher speed ATM PVCs are fully supported though. For standard serial links, AutoQoS is not supported at all on sub-interfaces. A quick litmus test to see if AutoQoS will work on your desired interfaces or not is to verify that the service-policy configuration is supported. If not, you’ll probably have to reconfigure some links.

AutoQoS will not work if an existing QoS configuration exists on an interface. Likewise, when you disable the AutoQoS configuration, any changes you may have made to the template after the initial configuration will be lost.

Bandwidth statements are used by AutoQoS to determine what settings it should use, so remember that after updating bandwidth statements in the future, you have to re-run the AutoQoS commands.

Making It Work

In the most standard situation, where VoIP isn’t performing as it was promised, the network admin can quickly save the day by running the following on the WAN interface:

interface Serial0
bandwidth 256
autoqos voip

If it’s the local network that needs tuning, the following can be run on Catalyst switches (if running Enhanced Images):

auto qos voip cisco-phone
auto qos voip trust

It really couldn’t be easier than that. For the WAN example, we told the router that interface Serial0 has 256 Kbps, and to enable VoIP QoS. The switch example is similar, for Cisco phones.

The neat part about this is that AutoQoS is actually doing more than just generating a configuration for you and forgetting about it. If you run the command show autoqos interface s0, you will see much more than just your standard old interface configuration. It will show that a Virtual Template “interface” has been created, and that a class is applied to the interface. The same output will also show you the configuration of the template and class-map, with an asterisk next to each entry that was generated by AutoQoS. It’s actually keeping track of what was done automatically so that you can learn what AutoQoS is doing. As mentioned previously, however, don’t forget that removing the AutoQoS configuration will destroy all QoS settings on an interface, not just the ones that AutoQoS configured.

Finally, remember to enable QoS on both sides of a WAN link to truly prioritize VoIP packets. Don’t forget to read through the Cisco documentation before deploying it, even though AutoQoS is simple, in comparison. It is simple, but the more prepared you are the easier it is to deploy.

Cisco will hopefully continue this trend of providing Auto features for complicated, but common tasks. AutoQoS for VoIP sure does enable a much larger audience to correctly deploy VoIP over a wide variety of networks.

When he's not writing for Enterprise Networking Planet or riding his motorcycle, Charlie Schluting is the Associate Director of Computing Infrastructure at Portland State University. Charlie also operates, and recently finished Network Ninja, a must-read for every network engineer.