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QOS - Untrusted port on 2950 Enhanced Image is not working

Hi everyone,

I am currently studying for QOS 642-642 exam and I have a problem with the untrusted boundary. I have 2 sets of 2950G-12-EI in my lab running on IOS 12.1(22)EA12. As per Wendel Odom exam certification guide and 2950 Software configuration guide on 12.1(22)EA11, all packets that are received on untrusted ports will be marked with COS 0 which then maps to DSCP 0.

However, when simulating this among Cisco IP Communicators and Cisco hard IP phones, I get totally different results. Inspecting the packets via Wireshark, I get RTP packets with the original DSCP value of EF and skinny packets with DSCP value of AF31.

The following are the typical configuration on all the ports

interface FastEthernet0/1

switchport mode access

switchport access vlan 100

switchport voice vlan 200

spanning-tree portfast

sh mls qos int fa0/1

trust state: not trusted

trust mode: not trusted

COS override: dis

default COS: 0

pass-through: none

trust device: none

The cos-dscp and dscp-cos mapping are at its original value.

Is there something wrong with the documentations, or my understanding on the subject or my equipments? Would someone able to shed some light on this matter? Thanks.


Re: QOS - Untrusted port on 2950 Enhanced Image is not working

on ur switch

on the interface level do the following commands:

CAT2950(config)#interface FastEthernet0/1

CAT2950(config-if)#mls qos trust cos


CAT2950(config-if)# mls qos trust device cisco-phone

good luck

if helpful Rate

New Member

Re: QOS - Untrusted port on 2950 Enhanced Image is not working


The issue I raised up here is not about trusting the cos or dscp received from the devices. I have no problem with that in my studies and simulating that in the lab.

However, the issue here is that the untrusted port is not doing what it is supposed to do as stated in the documentation and study guides. This is where it got me confused. It is like the switch never bother to set the COS and DSCP to 0.

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