mls qos trust dscp

[tedauction]

Hello, I have a policy-map on a switchport of my 2960 which is marking traffic as EF - confirmed working. However I am not sure if that marking is being stripped to 0 when it enters and leaves the trunk port of this switch. My trunk has no 'mls qos trust dscp' command, BUT when I run the command 'sh mls qos int gi0/1 stat' I see there is outgoing DSCP 46 traffic. So the question is, is my trunk keeping the DSCP 46 traffic or stripping it back to 0 ?

interface GigabitEthernet0/1 (note I have no trust DSCP command so does that mean all DSCP marked traffic is getting stripped to 0 when it enters/leaves this trunk port ?)
description TRUNK_TO_xxxx
switchport trunk allowed vlan 53,203,570
switchport mode trunk
switchport block multicast
no ip igmp snooping tcn flood
end

I run the command: 'sh mls qos interface gi0/1'

GigabitEthernet0/1
trust state: not trusted
trust mode: not trusted
trust enabled flag: ena
COS override: dis
default COS: 0
DSCP Mutation Map: Default DSCP Mutation Map
Trust device: none
qos mode: port-based

Then I run the command: 'sh mls qos int gi0/1 stat'. 
GigabitEthernet0/1 (All statistics are in packets)

dscp: incoming
-------------------------------

0 - 4 : 639692867 0 617 0 271
5 - 9 : 0 0 0 2 0
10 - 14 : 3966385574 0 0 0 0
15 - 19 : 0 0 0 0 0
20 - 24 : 0 0 0 0 860851
25 - 29 : 0 0 0 0 0
30 - 34 : 0 0 0 0 0
35 - 39 : 0 0 0 0 0
40 - 44 : 0 0 0 0 0
45 - 49 : 0 830 0 107 0
50 - 54 : 0 0 0 0 0
55 - 59 : 0 0 0 0 0
60 - 64 : 0 0 0 0
dscp: outgoing
-------------------------------

0 - 4 : 1888924085 0 0 0 0
5 - 9 : 0 0 0 0 0
10 - 14 : 1750596 0 0 0 0
15 - 19 : 0 0 0 0 0
20 - 24 : 0 0 0 0 0
25 - 29 : 0 0 0 0 0
30 - 34 : 0 0 0 0 0
35 - 39 : 0 0 0 0 0
40 - 44 : 0 0 0 0 0
45 - 49 : 0 138824494 0 2746669 0
50 - 54 : 12 0 0 0 0
55 - 59 : 0 0 0 0 0
60 - 64 : 0 0 0 0
cos: incoming
-------------------------------

0 - 4 : 2379648405 3956024234 0 1074089 24207
5 - 7 : 10832 1763 65748187

[Mark Malone]

Hi you should have mls qos trust dscp under the trunk interface to carry the qos marking

[Georg Pauwen]

Hello,

in theory, without 'mls qos trust' everything should be reset to 0. As I understand it, if you have 'mls qos' enabled globally, the switch will use the internal cos to dscp map, however, in the default map, nothing gets marked as DSCP 46...

Can you post the output of:

show mls qos maps cos-dscp

[Joseph W. Doherty · Accepted solution]

Trust only becomes a factor if QoS is globally enabled.

I recall (?) setting CoS/ToS to zero, only happens to ingress.

I also recall (?), you want a trust (or policy) for ingress traffic, if QoS enabled to avoid default CoS/ToS reset to zero.