Using quality of service (QoS) on Cisco network devices helps
provide both bandwidth and priority to certain types of network traffic. The
network administrator tells the network devices which traffic requires what
bandwidth and priority.
It’s important to understand the difference between
bandwidth and priority. As the network devices (switches or routers) encounter the
designated traffic, they give that traffic priority by sending it before other traffic; they give the
traffic bandwidth by sending more of it
than other traffic.
As I mentioned last time, configuring QoS is very complex.
There are many different ways of using QoS as well as different types of QoS. Last
time, I discussed how to use Cisco IOS AutoQoS to automatically configure a router
to give bandwidth and priority to VoIP traffic (“Learn
the benefits of Cisco AutoQoS”).
This time, let’s take a step back and look at how to
configure basic QoS for a given scenario using a downloadable template. Let’s
begin with a sample scenario.
Our sample scenario
Let’s say you have a Cisco 871W router at home. You use this
router for Skype VoIP service, to play Counter-Strike over the Internet, and for
traditional Internet activities (e.g., Web browsing and e-mail).
However, you’re experiencing performance issues with your Skype
phone service when simultaneously surfing the Web. When downloading a file,
your phone service sounds horrible. In addition, your Internet game suffers
when you download FTP files.
This is a prime example of how QoS comes in handy—you need
to give these different types of traffic the bandwidth and the priority they require.
Once you’ve configured QoS, you should be able to talk on the phone, play your
game, and download files—all at the same time—without experiencing any performance
issues.
Keep in mind that this is only an example. Once you
understand how to configure QoS, you can customize it to fit your organization’s
needs. In addition, the downloadable template will create the configuration
file, and you can modify it to fit your company’s needs.
Configure QoS
Let’s review the steps to configuring QoS on a Cisco router.
Step 1: Define the
traffic
You must tell the router which traffic you want to give QoS,
which you can accomplish either using an access control list (ACL) or using Network
Based Application Recognition (NBAR). An ACL is the traditional way to define
any traffic for a router.
With NBAR, however, the router just recognizes the traffic
traveling through the router—it knows that HTTP is HTTP, Skype is Skype, etc. But
there’s a limited
list of protocols and applications that the router recognizes.
While the router won’t recognize every single application, each
IOS upgrade adds more to the list. In addition, you can create custom
application recognition files.
Step 2: Create a class-map
A class-map defines the traffic into groups. For example,
you could create a class-map called VoIP
traffic and put all VoIP protocols under it.
Step 3: Create a policy-map
A policy-map matches the classes from the class-map with how
much bandwidth and/or priority you want to give this traffic.
Step 4: Apply the policy-map
to the interface
Like an ACL, you must apply the policy-map to the specific interface
you want it to affect. You can apply the policy-map in either output or input
mode. Here’s the command to use:
service-policy output|input {name of policy-map}
If you’re using NBAR to recognize the traffic, you must also
use the ip nbar protocol-discovery command
on the interface. This enables NBAR to begin looking at the traffic.
Download the QoS template
Now that you know the basic steps to configuring QoS, you
can get started with our downloadable QoS template. (This is the same template
that TechRepublic’s George Ou has used to explain how to configure the Cisco 871W
router for basic configurations, advanced
871W configurations, and an IPSec
site-to-site VPN.)
Follow these steps:
- Download
the template. - Open
the Excel file, and fill out the yellow sections on the Variables worksheet. - Click
the Replace button; it will generate the appropriate QoS configuration on
a new sheet called QOS-1. - Copy
the configuration from the Excel file, and paste it into the Cisco CLI. You
can copy directly from Excel into a Telnet or SSH session or even the
console port.
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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.
