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Looking for a way to debug QoS DSCP marking on C3750 switch

Level 1
Level 1


We have  DSCP marking setup on LAN switches across branch sites.

And we've found only a few queues are in use on WAN router at a branch site.

WAN routers are supposed to assign packet to defined queue depending on DSCP set on incoming packet.

We're guessing LAN switch, where DSCP marking is happening, is doing wrong DSCP marking somewhat as the cause of above issue.

Is there any best way to debug on LAN switch how LAN switch is marking DSCP to verify whether LAN switch is causing the issue? Or any best show commands to check it?

LAN switch model is C3750.

3 Replies 3


 The best way I can recommend is use port span (monitor session)  on the switch with Wireleshard and analize the packet in the wireshark. On the Wireshark you can have a lot of filter but for QoS making, will not be nacessary actually. Just expand the packet and you are going to see the QoS information.


Joseph W. Doherty
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I recall the 3750 has some ASIC/port commands that will show frame/packet counts, per DSCP markings, for both ingress and egress.

BTW, on a 3750 if QoS is enabled, it will, by default, reset ToS markings.

Level 1
Level 1


From CLI you can use the following command to check DSCP markings-

Switch#show mls qos int gi1/0/1 statistics


This will show you which dscp marking are increasing on the interface in both ingress and egress direction. This can give you some idea about traffic patten, for exact marking on any specific packet you can take SPAN and check it in Wireshark.




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