What happens to marked packets without qos trust on port?

[Mike Assel]

Hello.  Just wondering if the experts out there could confirm  something for me:  I have a switch that has been globally configured for  qos specifically for IP phones.  However one of the switchports I have  re-configured so it looks like this

interface FastEthernet0/36

description LAB

switchport mode access

switchport voice vlan 208

spanning-tree portfast

end

My  question is this: if a device that marks it's own packets (let's say  EF) is connected to this port, once the packets enter the port the  switch will remark them because there is no trust command configured on  the port, correct?  I would assume the default would be to zero?  The  reason I ask is that we have a network monitoring service that is setup  to monitor the qos markings of packets crossing our WAN and to notify us  if the packets get remarked.  I have confirgured the packets to be  marked EF on the device connected to the switchport mentioned above, so in my mind the markings should be getting changed and my service should see that, but it doesn't.  So I want to make sure I understand how it is supposed to work in the switch  before pointing the finger at the service.

[John Blakley · Accepted solution]

Mike,

The default behavior would be for the switch to set the tag to default CS0 because the port is considered untrusted, so you're correct.

Default Voice VLAN Configuration

The voice VLAN feature is disabled by default.

When the voice VLAN feature is enabled, all untagged traffic is sent according to the default CoS priority of the port.

The CoS value is not trusted for 802.1P or 802.1Q tagged traffic.

http://www.cisco.com/en/US/docs/switches/lan/catalyst3550/software/release/12.1_12c_ea1/configuration/guide/swvoip.html

HTH,

John

[paul driver]

Hi mike,

Your correct in your assumption.

If the maked  packet goes into a non enabled mls qos switch, the switch  will rewrite the marking to a cos 0

Switch#sh mls qos

QoS is disabled

QoS ip packet dscp rewrite is enabled

Switch#sh mls qos int fa0/1

FastEthernet0/1

QoS is disabled. When QoS is enabled, following settings will be applied

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

res

Paul

[Marwan ALshawi]

have you tired  to disable DSCP rewrite with global command "no mls qos rewrite ip dscp"

HTH