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Community Member

COS values help

I have read the following article about 3 time and I still cannot understand this stuff.

I cannot not figure out how COS works. I do not understand how, from the following config, the equipment knows what traffic should be in each class.


class-map match-all COS3

match ip dscp af21

class-map match-all COS2

match ip dscp af31

class-map match-all COS1

match ip dscp ef



policy-map COS_PROFILE_101

class COS1

  priority percent 40

  set ip dscp ef

class COS2

  bandwidth remaining percent 60

  set ip dscp af31

class COS3

  bandwidth remaining percent 30

  set ip dscp af21

class class-default


  set ip dscp default



I see the "match ip dscp af21" , but I see no ACL's that tells it what traffic it should match.

Could someone explain to me how the equipment knows what traffic is voip or video?

Super Bronze

Re: COS values help


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I see the "match ip dscp af21" , but I see no ACL's that tells it what traffic it should match.

That's because the match statement supports certain matching without using ACLs.  In this case, it examines the ToS byte, and if the value is a DSCP AF21, it matches.

Could someone explain to me how the equipment knows what traffic is voip or video?

In your policies, it doesn't really know what traffic is VoIP or video.  Your policy is just looking at certain ToS settings; this is assuming your VoIP and video traffic is already (correctly) tagged/marked.

Re: COS values help

As Joseph has already mentioned, you don't need to specifically configure an ACL to match for marking.

CoS or Class of Service values, are used on 802.1q trunks. This is because, when a switch tags a frame with 802.1q vlan tag, which includes within it a field called 802.1p for User Priority. This is where the CoS values come into play.

You can also specify an ACL, and have it mark frames based upon that value.

Also, within each IPv4 packet, there is a field called the ToS byte. This ToS byte is 8 bits (as you would expect)

Ths is where you can mark traffic with the following values, COS, DSCP, and IPP.

In the old days, you could only use 3 bits for marking, so this would give you 8 levels of marking.


So after a while, people realised we needed more ways to mark traffic, and decided to use 6 bits for marking. This is where the

ToS byte comes into play. You can have DSCP, COS, and IPP values.

COS, is backwards compatible with IPP, and COS uses the first 3 bits, and the last 3 are 000.

DSCP uses the 6 bits out of the ToS byte.

Community Member

COS values help


The QoS marking would be receiver from some other downstream devices.

For Eg: In your configuration I can see

class-map match-all COS1

match ip dscp ef

Most probable this would have come from a Cisco IP Phone. Cisco IP Phone marks voice traffic as EF and send to its uplinks.

Same way there should be some ACL in a Router or Switch downstream to mark the traffic, when the traffic is marked you can match them anywhere in the network. No need to classify in every hop.

Hope this clears your query...

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