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New Member

802.1P Question

I am confused on something. 802.1p is contained within an 802.1q tag at layer 2, I understand all this stuff. However MS workstations and the such can now mark 802.1p priorities, yet they are not connected to 802.1q trunks. I know some Cisco switches you can configure for .1p classification on the native or "untagged" vlan. What I dont understand is how .1p marking can be done on these end devices and not have their frame thrown away if other devices are unaware he is marking layer 2 CoS wth 802.1P ?

Is it safe to say, than workstations can tag with 802.1p even though they are not talking to a dot1q trunk on the switch, and as long as the switches in the path have QoS enabled they will pass the traffice and adhere as configured ??

Please help me clarify. Doc's I read try to seperate .1p and .1q while others state that .1p relies on the dot1q tag to inserts it's bits, which I believe. If that is true how can dot1p exist without dot1q simply put ?

New Member

Re: 802.1P Question


the dot1Q field is always present in the ethernet frame header even when there is no dot1Q trunk.

if a switch is unaware of COS it simply don't worry about the COS and treat the frame as a normal frame.

if the switch is QOS enable, it will treat the frame regarding the the COS and QOS policy.

it's also true to say that if workstation tag 802.1p in the frame, it can affect the network QOS policy if the network is not properly configured.

In fact this is probably one of the main reason why cisco now deliver the new 2950 and 3550 series family switchs. Those switch are now able to remap the 802.1p tag.

However, if you have workstation connected to Ip phone, the phone can remap the dot1P field for the data port on the phone.

New Member

Re: 802.1P Question

I understand all of that, but I disagree. A standard 802.3 ethernet frame does NOT contain a dot1q tag. You would have to enable dot1q before it would insert the tag