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Frequent Contributor

difference between cos and qos

can anyone define these, i am a little confused ?



Hi Carl,

Quality of Service is the capability of a network to provide better service to the selected network traffic by giving priority including dedicated bandwidth, controlled jitter and latency.

Where Class of Service is the value given in Layer2 frame to differentiate the traffice to give diferential Qos treatment. It is a 3 bit field to map the qos from layer 3 to layer 2.viceversa.

so are you saying QOS = how much you want to reserve bandwidth for a certain application, and COS is basically the type of application ? I am unsure ?

thanks for your help

Rising star

carl look at this link:

this link is having good explanation with the diagram...

rate this post if it helps



Hi carl,

QoS is 'how to manage my traffic', that is quality of service. QoS is essentially:

- classification: wich type of traffic?

- congestion management: how to manage a congestion

- congestion avoidance: how to prevent a congestion

- policing: drop after a threshold

- shaping: no drop, use buffers as possible

- signalling: reserve bandwidth for traffic

- link efficiency: fragmentation & interleaving

Class of Service (CoS) is a queuing discipline. An algorithm compares fields of packets or CoS tags to classify packets and to assign to queues of differing priority. Unlike Quality of Service (QoS) traffic management, CoS does not ensure network performance or guarantee priority in delivering packets.

Cos tag is a type of classification, like Ip Precedence, DSCP, QoS groups, CLP. You have Cos tags on 802.1p links (an extension of the IEEE 802.1Q VLANs tagging --> trunks).



Frequent Contributor

so what exactly is cos doing here ?

Cos is a traffic classification (802.1q trunks).

Devices like switches (3550, 4500, ...) use Cos to choise an output queue, and DSCP for other operations, like WRED and so on.

For example, see 3550:

"Different types of traffic are assigned to either threshold, which depends only on the internal DSCPs. This differs from the queue assignment, which only depends on the CoS of the packet."



Wilson Samuel
Rising star


The easiest and simplest form of definition would be to say that:-

1.CoS= L-2 QoS Specs

2.QoS= L-3 specs of QoS which is achieved either by setting DSCP or by IP Precedence.

Hope it solves the query.


Wilson Samuel

I am still unsure about this, Im sorry to trouble anyone, Is there any easier explanation ?

Hi Carl,

Could you put in your own words abou your own understanding about CoS and QoS?

I shall be glad enough to tell where to correct yourself.


Wilson Samuel

I could like to say it...

QoS is the general term that a device can classify the traffic which pass through it in order to make it deliver as predefined order, priority or behaviour.

CoS is the way to classify the traffic when there is QoS.


Bhishma Khanna
Cisco Employee


QoS is the Quality of Service feature by which you can allocate the bandwidth to specific type of traffic.

The question is, how to do this?

Answer is, we can do this on the basis of layer 2 information(COS) and layer 3 information(DSCP and IP precedence).

COS is the 3 bits long field which means the total values for COS could be 2^3=8, starting from zero

0 1 2 3 4 5 6 7

Cisco IP phones tag the voice traffic with COS value 5.

So, if in the switch connected to IP phones you want to apply QoS to separate your voice traffic then you may do it for COS value 5.


Hey Carl,

In the most laymen of terms CoS puts a tag on different types of network traffic like 0 for Video Streaming or 1 for VoIP where 0 is rated higher than 1, and QoS uses those tags to determine which traffic gets preferential treatment basically.

This is my understanding of the two, and anyone here is welcome to elaborate or correct me if I'm wrong.