QoS Classification and Marking

To use Quality of Service for a traffic, firstly traffic need to be identified. With this identification, traffic types are classified and then they are marked with an understandable way by the network. This process is basically called “QoS Classification and Marking“. Another important mechanisms are Qos Traffic Policing and QoS Traffic Shaping. We will talk about this in the next lesson.


The identification mechanism used by QoS can be divide into two important process. These process are :

  • Classification
  • Marking


Now, let’s go a little deeply on QoS Classification and Marking, and talk about these two important QoS Process more.



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Table of Contents

QoS Classification

There are carious of traffic types in a network. These traffic types can be data, voice, video streaming etc. Without QoS, all these traffic types are behaved similarly. But behaving similarly to all the traffic types is not a proper way. Because, different traffic types need different threatments.


For example, voice traffic must be fast but security is in the second plane for voice traffic. Beside, pure data, ftp can be slower than a voice traffic.


Because of the fact that different types of traffics need different threatments, first of all we need to categorize the type of our traffic. Identifying and categorizing the type of the traffic is called “Classification”. After this process, we know that, we have a voice traffic, or video, or what else.


QoS Classification Process can be done by checking the different fileds of a packet. There are fields that shows the traffic types in a packet like IP Precedence,DSCP. Beside, incoming interfaces, source and destination addresses can also be used for Classification.


QoS Classification is done close to the source. This is because, early determination of the type and threat as required through the network.


QoS Marking

After classification, traffic type determination must be showed also in the packet. To do this, a field in a packet header is changed. This changes explains that, the packet is belong to a specific type of traffic. The name of this process is called “Marking”. It is also called “Coloring”.


QoS Classification Process is a must before Marking Process. Because, you can not mark something about its characteristic without knowing what is it.


Marking can be done in different levels with different field changes. With this changes, the traffic is quickly recognized anywhere in the network.


In Layer 2, in Ethernet Header, Class of Service field is used for Marking.

In Layer 2.5, in MPLS Header, Type of Service (Experimental) field is used for Marking.

In Layer 3, in IP Header, Type of Service IP Precedence and DSCP fields are used for Marking.

In Upper Layers, NBAR and Deep Packet Inspection are used for Marking.




In Cisco CCNA and CCNP ENCOR Certification Courses, Traffic Classification and MArking is an important QoS Lesson.


Lesson tags: QoS, classification, marking, Quality of service
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