QoS and data packet

[fabio.marino]

Hi to all.

I have a topology like:

switch->ip phone->PC

Swicth configuration is:

Interface fa0/4

  switchport voice vlan 10

  mls qos trust cos

  mls qos trust device cisco-phone

  switchport priority extende cos 0

In brief, based on my understood:

- Voice will be transmitted on vlan 10 with a default CoS equal to 5. This is true only if a cisco-phone is connect with the Fa0/4 of the switch, otherwise will be used the default CoS value (usually 0)

- Ip Phone override the CoS of packet come from the PC with a CoS equal to 0

Following my doubt:

1)Is it correct my resume?

2) Is not  correct to say that the port Fa0/4 of the switch will trust the priority for the frames received from the PC? I think that this is true, because the trust boundaries in this case is the ip phone.

3)What happen in the case of voice and data using the same vlan (and so the same subnet)?

Thanks in advance, Fabio.

[dixho]

Switches have no control on the CoS value used by the phone. If you configure mls cos trust cos and mls qos trust device cisco-phone, the switch does not modified the CoS value of the traffic on the voice VLAN.

From the configuration, the data device is supposed to be on native VLAN. Cisco switches assume that no 802.1p/q tag is added to native VLAN. Thus, there is no CoS value of the switch. By default, Cisco switches assign CoS value to all traffic on data vlan. You can change the CoS value of traffic on data vlan by using the command "switchport priority extend cos"

As far as putting data and voice traffic on the same VLAN, it is not a behavior of Cisco IP phones. Assume that you use a third party device, which put both voice and data traffic on the same VLAN, the switch will convert all traffic on voice VLAN to CoS 0 or whatever value set on switchport priority extend cos because the switch does not receive CDP from the phone port.