Quality of Service (QoS) Classification and Marking

Quality of Service (QoS) is a mechanism or technology that handles network traffic and allocates capacity to ensure the performance of critical applications. All QoS mechanisms are designed to resolve or reduce bandwidth overutilization, delay, flapping, and packet loss in a network. Some of the QoS tools are Classification and Marking, Queuing, Policing and Traffic Shaping, and Congestion Management.

qos classification

QoS Classification

Before we can configure any QoS tools, like queuing, policing, or shaping, we have to look at the traffic that is coursing through our network device and identify it first. QoS classification refers to the process of classifying the type of IP packets or traffic. Traffic types can be data, video, or voice traffic. Traffic classes are the categories of traffic that are grouped based on their similarity.


QoS classification can be associated with a variety of match criteria such as:

  • Physical input interface
  • Source and destination MAC address
  • Class of Service (CoS)
  • IP Precedence value
  • Differentiated Services Code Point (DSCP) values
  • Source and destination IP address
  • Protocol type
  • Application type


Here are the CoS values (layer 2 header) and their applications:

7Network Control
6Internetwork Control
3Call Signaling
2Transactional Data
1Bulk Data
0Best Effort


QoS Marking

After classification of IP packet headers based on their contents, QoS Marking includes setting some bits inside a data link or network layer header, with the intention of letting other devices’ QoS tools classify traffic based on the marked values.

Marking can be done at different levels like Ethernet header (layer 2), MPLS label (layer 2.5), IP packet header (layer 3), Network Based Application Recognition (NBAR), and deep packet inspection (layer 7).

In other WAN technologies, we can use single-bit fields in Frame Relay and ATM networks to mark a frame for Layer 2 QoS. Frame Relay determines the discard eligibility (DE) bit setting, and ATM defines the cell loss priority (CLP) bit.

DSCP is the more preferred QoS tool because the router can quickly get information from a single byte on the IP header. Also, the IP header doesn’t change between source and destination addresses.


Here are the DSCP values (layer 3 header) and their equivalent applications:

DSCP ValueApplication
CS7Network Control
CS6Internetwork Control
CS5Broadcast Video
AF4Multimedia Conferencing
CS4Realtime Interactive
AF3Multimedia Streaming
AF2Transactional Data
AF1Bulk Data
DFBest Effort


QoS Trust Boundary

The idea behind the QoS trust boundary is to avoid end devices manipulating traffic prioritization. Once we include a network device in the QoS trust boundary (preferably at the access layer), there’s no need to remark any packets, and this network device will handle QoS marking.

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