Hi all, i went on a course last week, he said the layer 2 switches are layer 3 aware for qos? what does that mean, does that mean they can mark the dscp value in the packet, or would they still mark the 802.1p value, also what value do phones mark dscp or 802.1p ?
What the instructor said is essentially correct, but only applies to more recent Layer-2 switches. For example a Catalyst 2950 running the EI features supports Layer-3 QoS - that is it can read and re-write the DSCP values in the ToS field on ingress ports (the 2950EI doesn't support all 64 DSCP values, only 0, 8, 10, 16, 18, 24, 26, 32, 34, 40, 46, 48, and 56).
The 3550 (I know it is layer-3) and all later switches support all DSCP values - I am thinking more specifically about the 2960 here. Obviously what the switch can do with these values depends on the platform - generally the DSCP value will map to an egress queue (or possibly back to it's CoS value and then to a queue). There are lots of platform limitations between the various Catalyst platforms so read the documentation to understand what you can and can't do.
Carl, please read the documentation on CCO. Regardless of the switch platform the explanations at the beginning of the QoS section in the documentation for each switch is generic. Read this to understand what QoS is and what it's uses are:
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