What if you wanted to enable QoS for your new VoIP phones on your network? And after reading the QoS chapter, you've decided that QoS is fairly confusing and you now have doubts about deploying it. What if I told you that you could deploy QoS with one simple command? Perhaps you don't believe it because it sounds too good to be true. Well, in some ways, you might be right. But if your network only calls for VoIP QoS, AutoQoS might be exactly what you are looking for.

AutoQoS is Cisco's answer to the growing complexity of QoS configurations. With one command on your interfaces, you can enable VoIP QoS automatically on low-speed (768 kbps or lower) interfaces. Okay, it's not just one commandyou need to use the bandwidth command as well.

 interface Serial1
 bandwidth 256
 ip address
 auto qos voip

With AutoQoS, you get the following features:

  • Dedicated bandwidth for VoIP
  • Automatic congestion avoidance and congestion management
  • Traffic priorities for certain network applications
  • Traffic shaping that conforms to inconsistencies in ingress/egress speeds

11.7.1. What Does AutoQoS Enable?

What is AutoQoS doing behind the scenes? For the complete details, see Cisco's page on this topic (, but here is a quick overview that hits the highlights, paraphrased from that page.

AutoQoS classifies voice trafficthat is, traffic with a real-time protocol and audio codec payload, as VoIP traffic. It then marks the traffic with a high-priority class of service (DSCP EF) and marks VoIP signaling traffic as AF31. Further, AutoQoS puts VoIP traffic into a strict priority LLQ with guaranteed bandwidth. VoIP control traffic is put into a nonpriority queue with a minimum bandwidth guarantee to ensure that there is no packet loss.

What happens to all the other traffic? To further differentiate VoIP, all other traffic is downgraded to best effort (except for important items like routing protocol updates). If traffic is not marked DSCP EF or AF31, AutoQoS marks it as DSCP 0, which, as we mentioned earlier, is synonymous with best effort.

By putting VoIP traffic on the fast track, so to speak, you get good performance for VoIP traffic with little effort expended in tuning QoS for your site. Your needs may be more complex than this, but AutoQoS is certainly worth a try.

11.7.2. AutoQoS Command Usage

To enable AutoQoS, your network must have the following:

  1. Cisco devices at both ends of your link.
  2. Interface with a "low-speed" link (768 Kbps or lower).
  3. An IP address configured on the interface.
  4. The bandwidth command configured for the interface.
  5. Cisco Express Forwarding (CEF) must be enabled.
  6. Cisco Discovery Protocol (CDP) must be enabled. (It's enabled by default so unless you disabled it, you don't need to worry about it.)
  7. AutoQoS needs to be enabled on both sides of the link. Command syntax

The AutoQoS command options are:

 auto qos voip [trust] [fr-atm]

The optional keyword TRust tells the device to trust the DSCP packet markings when classifying traffic for QoS. By default, the device does not trust DSCP markings and uses NBAR for identifying VoIP traffic.

The other optional keyword fr-atm is used on frame relay-to-ATM internetworking. This command only applies to low-speed frame relay links.

In most cases, simply apply the auto qos voip command to both sides of a low-speed link (768 Kbps or lower), and you're done. AutoQoS discovery

Starting with 12.3(7)T, Cisco implemented an auto discovery feature that extends AutoQoS beyond just VoIP. By running AutoQoS in discovery mode for a few days (or weeks), the router collects traffic utilization via NBAR. The data that is collected is used to produce more advanced AutoQoS settings.

To enable AutoQoS discovery, use the auto discovery qos command on the interface. As data is collected, you can view the collection progress with the show auto discovery qos command.

 interface Serial1
 auto disovery qos

Once you are satisfied that enough time has passed to collect enough data to auto produce a relevant QoS policy (a few days at a minimum), you stop the collection with the no auto discovery qos command and then apply the new policy with the auto qos command.

 interface Serial1
 ! disable collection
 no auto discovery qos
 ! start auto qos process
 auto qos AutoQoS show command

To display the configurations created by AutoQoS, use the command show auto qos . The output of this command can be very verbose depending on the number of interfaces and types of hardware on your device.

Getting Started

IOS Images and Configuration Files

Basic Router Configuration

Line Commands

Interface Commands

Networking Technologies

Access Lists

IP Routing Topics

Interior Routing Protocols

Border Gateway Protocol

Quality of Service

Dial-on-Demand Routing

Specialized Networking Topics

Switches and VLANs

Router Security

Troubleshooting and Logging

Quick Reference

Appendix A Network Basics


Cisco IOS in a Nutshell
Cisco IOS in a Nutshell (In a Nutshell (OReilly))
ISBN: 0596008694
EAN: 2147483647
Year: 2006
Pages: 1031
Authors: James Boney © 2008-2020.
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