CoS and DSCP relationship

[computerone1]

Catalyst 2950 / Version 12.1(22)EA12 / Running Enhanced Image

Hi

I'm very puzzled about the CoS field (Layer2) and the DSCP field (Layer3) relationship.

I've setup a CoS7 (set cos 7) on a Catalyst 2950 access port, and started SPANning the trunk port. It appears that simultaneously:

. The frames 802.1q headers get a CoS7

. The IP headers get a DSCP 56

I Wonder about these questions:

. Does applying a CoS value automatically implies that the IP header is modified with the coresponding (show mls cos maps) DSCP value?

. As a Catalyst 2950 doesn't allow to use 'set cos' in Policy-maps, is it okay to use 'set ip dscp 56' instead ? ( SPANning the trunk shows that in this latter case, the frames do get their CoS7 802.1q header applied)

Thanks for any input, please move this question to the relevant thread if I misplaced it !!

Oliver  

[acampbell]

Hi,

Look at the output from show mls qos maps cos-dscp.
The default are shown below. --- So COS7 == DSCP 56

MYSWITCH#sh mls qos map cos-dscp
   Cos-dscp map:
        cos:   0  1  2  3  4  5  6  7
     --------------------------------
       dscp:   0  8 16 24 32 40 48 56

I cant find a valid link for your IOS 12.1.22EA12 config guide as the product is EOS/L as far as Cisco are concerened but this one will be very similar

http://www.cisco.com/c/en/us/td/docs/switches/lan/catalyst2940/software/release/12-1_22_ea2/configuration/guide/2940scg/swqos.html

Regards
Alex

[Joseph W. Doherty]

Does applying a CoS value automatically implies that the IP header is modified with the coresponding (show mls cos maps) DSCP value?

That depends on the device and what it's configure to do (perhaps by default).  Nothing per RFCs requires it.  (In fact, CoS and ToS usage are totally optional, and RFCs for their usage is really "suggestions".)

[computerone1]

Thank you for your inputs and comments, that puts things into perspective.

I'll investigate the issue on a Catalyst 2960+ to have a look at a more recent piece. :-)