Simple call center

3750 stack ------ 3945

On the 3945 there is a service policy, matching a few different classes. One class is voice traffic (ip dscp ef (46)).

My confusion lies on the 3750 stack. As there are no QOS configurations added. So per my understanding, even if a IP phone was to mark its traffic with a COS of 5, this would be over-written by the switch with a COS of 0.

If I do a "show mls qos interface" this is what all interfaces show on the 3750 stack.

GigabitEthernet2/0/X

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

On the 3945 I see traffic hitting the voice class-map

ROUTER#sh policy-map interface serial 5/0

Serial5/0

  Service-policy output: CBWFQ-OUT

    queue stats for all priority classes:

      queue limit 64 packets

      (queue depth/total drops/no-buffer drops) 0/0/0

      (pkts output/bytes output) 1264670357/81494676622

    Class-map: VOICE-CALL (match-all)

      1263827431 packets, 81233688108 bytes

      5 minute offered rate 273000 bps, drop rate 0 bps

      Match: ip dscp ef (46)

      Priority: 3500 kbps, burst bytes 87500, b/w exceed drops: 0

So I am confused how this is working. Could anyone explain this to me?