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QoS Preservation of markings between two end systems

I have a requirement to preserve markings end to end across a network utilising 3750X switches, I am marking the packets without a problem, but due to the rewriting function the marking is being overwritten by ther COS-->DSCP maps.

To simplify the scenario I have set up a test environment (see Diagram) and am using ICMP as a simple test

using ICMP to test with

Both 3750 have 12.2(35)SE5


mls qos

mls qos rewrite ip dscp

ip access-list extended ICMP

permit icmp any any

Class-map ICMP

match access-group name ICMP

policy-map TEST

class ICMP

set dscp 46

int g1/0/1

switchport mode access

swicthport access vlan 100

spanning-tree portfast

mls qos vlan-based

int vlan100

ip address x.x.x.x

service-policy input TEST

I have used wireshark to verify that the packets are being marked, with the' mls qos rewrite ip dscp' they are, without rewriting does not occur.

Moving my sniffer to the 3750-2 I am monitoring the Layer3 connection (marking preserved), however on the layer 2 trunk the marking has been reset to 0 , if I remove the 'mls qos rewrite ip dscp' from 3750-2 then I see the marking on the layer 2 trunk, however I am then unable to remark anything generated on this switch. I believe with the 'mls qos rewrite ip dscp' enabled the L3 to L2 transistion removes the IP DSCP sets the COS to 0 and themnthe COS to DSCP mapping ensures the DSCP is set back to 0.

I have tried implementing additional marking policies on 3750-2 but to no avail.

What I need is assistance ensuring that that a frame / packet marked in 3750-1 can keep it's marking to the end system over layer 3 and a layer 2 trunk. The only place I wan t the marking to be stripped off is when the dot1q tag is removed as it goes through an access port.

Any help is very much appreciated.


QoS Preservation of markings between two end systems


Not sure how to answer your question, but I did find this good article on the subject:



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