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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 ??

4 REPLIES
New Member

Re: 802.1P Question

802.1p uses an unused 3-bit field in the 802.1q VLAN tag. When a switch not configured for 802.1q receives a frame tagged for 802.1q I think it just discrards the 802.1q tag not the enitre frame because the tag is appened to end of the MAC frame and the switch is not looking for the appended VLAN tag.

New Member

Re: 802.1P Question

Or for a device not intelligent enough, or not configured to do so, could just throw the frame away if it is carrying a max payload. The frame could be viewed as a giant. The 3 bit are not unused bits in the VLAN tag, remember, it's the 802.1P standard, those 3 bits are designated, it would be safer to say that dot1q does not rely on those bits, nor does it need them.

d-

Re: 802.1P Question

.1p and .1q are two separate issues.

The .1p uses three bits in the IP header to differentiate the CoS. So .1p resides on layer three. 802.1q is a tagging mechanism, which alters a frame to identify the VLAN to which it belongs when the frame is transmitted over a trunk-link. Just like ISL encapsulation, this is a layer two issue.

New Member

Re: 802.1P Question

No you are wrong, 802.1p resides at layer 2 in the dot1q header, hence the "802" committie header, you are thinking of the TOS bits in the IP header which very much resemble them. Dot1p is very analagous to IP Prec, layer 2 and layer 3 respectively.

-d

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