Cisco CCNP Certification / BCMSN Exam Tutorial: Writing QoS Policy

QoS - Quality of Service - is a huge topic on both the BCMSN exam and real-world networks. QoS is so big today that Cisco's created separate specialist certifications that cover nothing but QoS! It can be an overwhelming topic at first, but master the fundamentals and you're on your way to exam and job success.

If you work with QoS at any level - and sooner or later, you will - you've got to know how to write and apply QoS policies.

Creating and applying such a policy is a three-step process.

1. Create a QoS class to identify the traffic that will be affected by the policy.

2. Create a QoS policy containing the actions to be taken by traffic identified by the class.

3. Apply the policy to the appropriate interfaces.


If the phrase "identify the traffic" sounds like it's time to write an access-list, you're right! Writing an ACL is one of two ways to classify traffic, and is the more common of the two. Before we get to the less-common method, let's take a look at how to use an ACL to classify traffic.

You can use either a standard or extended ACL with QoS policies. The ACL will be written separately, and then called from the class map.

SW1(config)#access-list 105 permit tcp any any eq 80

SW1(config)#class-map WEBTRAFFIC

SW1(config-cmap)#match access-group 105

Now that we've identified the traffic to be affected by the policy, we better get around to writing the policy! QoS policies are configured with the policy-map command, and each clause of the policy will contain an action to be taken to traffic matching that clause.

SW1(config)#policy-map LIMIT_WEBTRAFFIC_BANDWIDTH

SW1(config-pmap)#class WEBTRAFFIC

SW1(config-pmap-c)#police 5000000 exceed-action drop

SW1(config-pmap-c)#exit

This is a simple policy, but it illustrates the logic of QoS policies. The policy map LIMIT_WEBTRAFFIC_BANDWIDTH calls the map-class WEBTRAFFIC. We already know that all WWW traffic will match that map class, so any WWW traffic that exceeds the stated bandwidth limitation will be dropped.

Finally, apply the policy to the appropriate interface.

SW1(config-if)#service-policy LIMIT_WEBTRAFFIC_BANDWIDTH in

Getting your CCNP is a great way to boost your career, and learning QoS is a tremendous addition to your skill set. Like I said, learn the fundamentals, don't get overwhelmed by looking at QoS as a whole, and you're on your way to success!

Cisco CCNP / BCMSN Exam Tutorial: Switches, QoS, And Cisco's Networking Model

QoS is a big topic on your BCMSN and CCNP exams, and for good reason. As more and more traffic flows through today's networks, accurately applying QoS to both your routers and switches becomes more important.

Note the phrase "accurately applying". You must have a plan in place before you start configuring QoS on your switches, and to create such a plan you should use Cisco's Three-layer Hierarchical Model.

This model breaks switches down into three main groups - Access, Distribution, and Core. You're familiar with these groups from your CCNA studies, and now you've got to apply this knowledge.

The QoS workload should be borne by the Access and Distribution layers, because the Core layer switches need to be left alone as much as possible to their primary purpose - switching!

Traffic should generally be classified and marked at the Access layer. This allows traffic to be assigned the desired QoS values and carry that value throughout the network.

If you choose to change CoS-DSCP mappings, this will generally be done at the Distribution layer. Since distribution layer switches will be receiving frames and packets with QoS values from the access layer switches, the appropriate "trust" and "no trust" statements should be configured on the appropriate distribution layer switches.

Any traffic received by core switches should already be classified and marked as needed. The key with core switches is to use a simple queuing setup to keep the switching process fast. Fast, fast, fast!

Real-world note - Low Latency Queuing (LLQ) is an excellent choice for core switches. The name says it all - low latency! The configuration of LLQ is not a BCMSN topic, but a quick search on the term low latency queuing will quickly bring up several Cisco LLQ configuration documents.

Knowing the three layers of Cisco's networking model and the basic QoS operation and commands is vital to passing the CCNP exams, but even more importantly, you've got to apply this knowledge carefully and accurately to make QoS work for you in today's production networks.

CCNP Certification / BCMSN Exam Tutorial: QoS Service Types

To pass the CCNP exams, you’ve got to master Quality of Service, and the first step in doing so is knowing the differences between the different QoS types.

Now this being Cisco, we can't just have one kind of QoS! We've got best-effort delivery, Integrated Services, and Differentiated Services. Let's take a quick look at all three.

Best-effort is just what it sounds like - routers and switches making their "best effort" to deliver data. This is considered QoS, but it's kind of a "default QoS". Best effort is strictly "first in, first out" (FIFO).

An entire path from Point A to Point B will be defined in advance when Integrated Services are in effect. Integrated Services is much like the High-Occupancy Vehicle lanes found in many larger cities. If your car has three or more people in it, you're considered a "priority vehicle" and you can drive in a special lane with much less congestion than regular lanes. Integrated Services will create this lane in advance for "priority traffic", and when that traffic comes along, the path already exists. Integrated Services uses the Resource Reservation Protocol (RSVP) to create these paths. RSVP guarantees a quality rate of service, since this "priority path" is created in advance.

Integrated Services is defined in RFC 1613. Use your favorite search engine to locate a copy online and read more about this topic. It's a good idea to get into the habit of reading RFCs!

Of course, if you've got a lot of different dedicated paths being created that may or not be used very often, that's a lot of wasted bandwidth. That leads us to the third QoS model, the Differentiated Services model. Generally referred to as DiffServ, there are no advance path reservations and there's no RSVP. The QoS policies are written on the routers and switches, and they take action dynamically as needed. Since each router and switch can have a different QoS policy, DiffServ takes effect on a per-hop basis rather than the per-flow basis of Integrated Services. A packet can be considered "high priority" by one router and "normal priority" by the next.

Believe me, this is just the beginning when it comes to Quality of Service. It's a huge topic on your exams and in the real world's production networks, and as with all other Cisco topics, just master the fundamentals and build from there - and you're on your way to CCNP exam success!