i wondering if someone can help clarify the concepts of qos marking and trust. i have a 3524 switch connected to a 3550, which in turn connects to a multilayer 6509. the 3550 is doing ip routing as well. my thought is that i can mark the packets at the 3524pwr and then trust the cos coming into the 3550 from the layer 2 3524's. i think i'm solid on that.
then i get a bit confused. i think the 3550 maps the cos internally to a dscp value and then the frame that exits the 3550 has both values (cos and dscp) set. so does this mean that i can just trust this dscp value coming into the 6509?
i guess my main confusion comes when multilayer switches are involved--i am not sure whether to trust cos or dscp or if i could trust either value. i know that strict layer 2 devices will only look at cos but i'm not sure about the multilayer devices.
one more question--does the cos to dscp mapping get done in the 3550 if it is only acting as a layer 2 switch or is this only done when i enable routing?
any clarification will be appreciated. thanks