07-16-2013 09:55 PM - edited 03-04-2019 08:28 PM
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?
07-16-2013 10:16 PM
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
07-16-2013 11:55 PM
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
07-17-2013 02:50 AM
Disclaimer
The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.
Liability Disclaimer
In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.
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.)
07-17-2013 04:25 AM
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
07-17-2013 06:06 AM
Hi John, what in particular are you wondering about?
Discover and save your favorite ideas. Come back to expert answers, step-by-step guides, recent topics, and more.
New here? Get started with these tips. How to use Community New member guide