auosauos Member Posts: 186
I read in (QoS Volume 1).
Because of the CoS=5, the frame arrives at the Cisco Catalyst 2950 switch, it is instantly recognized as high-priority frame (CoS=5) and immediately placed in high-priority, no-delay queue. Because the switch recognize the frame as CoS=5, it is re-mark the DSCP filed to 40.

Ok, my question is how switch can touch the IP header and change DSCP to 40, the switch suppose to be Layer-2 device and cannot change the IP header.


  • shednikshednik Member Posts: 2,005
    The switch itself is only functioning as a layer 2 bridge yes, but that doesn't mean it can't read IP header information. I'm not that familiar with the QoS on a 2950 since I haven't set it up on one before but it's sounds like there is a policy map setup to match on CoS 5 and remark the packet with DSCP 40. Nothing out of the ordinary just depends on what the admin agrees to trust. I'm far from a QoS expert however but just my thoughts.
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