Configuring quality of service (QoS) can be very complex. The
Cisco IOS features many different methods for QoS—so many that there are entire
books that cover the topic of QoS.

To help make network administrators’ lives easier, Cisco has
developed AutoQoS, which configures QoS
settings for you. The concept almost sounds too good to be true, but is it? Let’s
review AutoQoS to see how it really works.

Review the basics

Let’s start off by reviewing the basics. QoS offers the
following benefits:

  • Prioritize
    traffic to delay sensitive traffic—for example, to ensure that lower
    priority traffic doesn’t damage voice traffic.
  • Prioritize
    traffic to ensure a less-critical application doesn’t slow down business-critical
    applications.
  • Prioritize
    traffic to ensure that undesirable traffic doesn’t overutilize bandwidth.
  • Conserve
    bandwidth by slowing the transfer of non-critical data.

You can configure QoS using a number of different methods on
a Cisco IOS router. Here are some of your options:

  • Manually
    configure QoS by creating ACLs to identify traffic and then control that
    traffic with QoS commands.
  • Use
    Cisco’s Security
    Device Manager (SDM)
    and the QoS Wizard to create predefined QoS
    policies, and then edit those policies.
  • Use
    AutoQoS to create policies based on real-time traffic flowing through your
    router or switch.
  • Use
    AutoQoS to create predefined voice over IP (VoIP) policies for VoIP
    traffic flowing through your IOS device.

Understand the benefits of AutoQoS

AutoQoS runs on Cisco IOS routers series 2600 through 7200,
and it runs on most Cisco routers with IOS version 12.2(15)T and later. AutoQoS
offers the following benefits:

  • You
    don’t need to understand QoS as much as you would if creating QoS policies
    from scratch.
  • You
    can modify the QoS polices you’ve created and use them elsewhere, like a
    template.
  • You
    can save time in QoS configuration.

Before running AutoQoS commands, you should enable CEF using
the ip cef command when in Global Configuration Mode. In addition, you need bandwidth
statements on your interfaces. This is because AutoQoS uses the bandwidth
statements when configuring bandwidth limitations for protocols it will be
prioritizing. If you change your bandwidth statements after configuring AutoQoS,
you must rerun AutoQoS.

Also, keep in mind that you don’t configure AutoQoS in
Global Configuration Mode—you do it in Interface Configuration Mode. Now, let’s
look at an example for how to configure AutoQoS for VoIP.

Configure AutoQoS

Configuring AutoQoS is easy—the hard part is understanding
what it configured for you, modifying the configuration if necessary, and
testing it to make sure it works. For our example, we’ll configure AutoQoS for
VoIP.

AutoQoS VoIP runs on certain types of interfaces. The most
simplistic example is a point-to-point T1 circuit between two routers with VoIP
traffic traversing that link.

Let’s say that we’re configuring AutoQoS on the Serial
interface between these routers. To begin, go into Interface Configuration Mode
for the interface that faces the source of the traffic you want to control. Here’s
an example:

Router(config)# interface Serial0/0
Router(config-if)# auto qos voip

And that’s it! The amount of commands that this one command
generates is surprising.
Listing A
offers a look at the part of this router’s show
running-config
affected by this command.

As you can see, the auto
qos voip
command is still on the
interface, but there’s been the addition of a policy called AutoQoS-Policy-unTrust. The untrusted
version of the command is the default.

Alternatively, we could have run the auto qos voip trust command. The trusted version of the command
uses Differentiated Service Code Point (DSCP) marking to prioritize traffic.
The version that we ran uses Network Based Application Recognition (NBAR) and access
control lists to identify the VoIP traffic we’re trying to control.

The new AutoQoS configuration generates many new commands,
but don’t let them confuse you. If you can see these commands working together
in the system, these commands don’t seem as daunting. Here’s a basic breakdown:

  • The
    access control lists define traffic.
  • The
    class-maps put the traffic into classes. They know which traffic to put
    into each class by the access control list that it matched, the DSCP bit
    that it matched, or the NBAR protocol that it matched.
  • The
    policy-map assigns priorities to the classes.
  • It
    applies the policy-map to the interface, which affects the traffic.

Now that we’ve run these commands on one side of the interface,
we also need to run them on the other side (i.e., the other router) as well. I
highly recommend running these commands in a test environment prior to using
them on a production network.

For more information on AutoQoS, check out these Cisco resources:

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David Davis has worked
in the IT industry for 12 years and holds several certifications, including
CCIE, MCSE+I, CISSP, CCNA, CCDA, and CCNP. He currently manages a group of
systems/network administrators for a privately owned retail company and
performs networking/systems consulting on a part-time basis.