CoS and Ethernet and Trunking Question

[mmedwid]

I've been doing some Cisco training and question came up "Is CoS supported in Ethernet?"  I had presumed so

but the answer said that CoS was only supported on 802.1q and ISL trunks.  So my question is - if you configure

"auto qos" on an access switch port (say a 3750POE) and phone VLAN with a phone attached what is the

effect on the frames?  Or is it the case that auto qos in this case is working at layer 3 and modifying the

DSCP first three bits?  Thank you for clarifying!

[Jon Marshall · Accepted solution]

An access port will accept a tagged frame if -

1) the vlan ID is set to 0 (which is what the phone actually does)

or

2) the vlan ID is set to the vlan the access port has been assigned to

So the phone will send tagged frames (with vlan ID = 0) to the switch. The 802.1p (CoS) markings will be contained within the 802.1q tag.  The phone will also set the DCSP marking in the packet as well.

Jon

[mmedwid]

Thanks much Jon.

[cadet alain]

Hi Jon,

needing a clarification: isn't  CDP telling the VLAN the phone is a member of and then when the phone sends frames they are tagged with this VLAN id? all of this if we set the auxiliary vlan with the switchport voice vlan "id" command and if we configure sw voice vlan dot1p then the VLAN id will be zero?

Regards.

Alain.

[Jon Marshall]

Alain

You are correct. CDP is used with Cisco phones and the auxiliary vlan. I was answering from a more generic perspective, probably because my only experience of VOIP has been with Nortel !!, so in that case it was vlan ID = 0 etc. But in the case of a cisco phone yes CDP is used to sort all this out.

Jon

[cadet alain]

Hi Jon,

Thanks for info as I've never worked with phones other than Cisco ones.

Regards.

alain.