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New Member

DSCP or COS

Can someone please clarify something for me.

When I am performing qos on a 3750, what is the difference between using DSCP and COS?
Why would I use COS?

5 REPLIES
New Member

DSCP or COS

Hi Luke,

CoS is a field that is used in an Ethernet header when an 802.1Q VLAN tag is present at the data link layer (layer 2), whilst DSCP forms part of the IP header (layer 3). With this in mind you would generally match/set COS values on Layer 2 links like trunks and DSCP on access ports or Layer 3 links.

Cheers

Brett

Cisco Employee

DSCP or COS

Hello,

In addition to Brett's response, you may be interested in reading the following two threads - a similar question arises quite often.

https://supportforums.cisco.com/message/3756172#3756172

https://supportforums.cisco.com/message/3981269#3981269

Best regards,

Peter

Super Bronze

Re: DSCP or COS

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Posting

You use COS when DSCP isn't supported on the device.  "True" L2 switches are unable to "see" the L3 ToS containing ToS.

Most current Enterprise L2 switches are often "enhanced", and a common enhancement is they can "see" the L3 ToS.

Given a choice, use DSCP.  (NB: it's not uncommon for some devices to use both.  For example, many VoIP phones will mark a VLAN tagged frame with COS 5 while also marking the L3 packet with DSCP EF.)

Re: DSCP or COS

Joseph, I often wondered about that

Most current Enterprise L2 switches are often "enhanced", and a common enhancement is they can "see" the L3 ToS

Super Bronze

Re: DSCP or COS

Hi John, what in particular are you wondering about?

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