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?
That is pretty much correct, you want to trust dscp on the 6509 after setting the trust boundary on the 3524. Here is a quote from CCO on the 35xx switch...
The Cisco AVVID features in the Catalyst 2900 and Catalyst 3500 series, with a minimum Cisco IOS release of 12.0(5)XU, allow interaction with the IP phones for extending the CoS marking rules. In addition, the Catalyst 2900 XL and 3500 XL switches can classify untagged packets at the ingress ports by setting a default CoS priority for each port. However, these switches (except for the 3548 XL) cannot reclassify any tagged packets, and they honor only the 802.1p priority and place the packets in the appropriate transmit queue. All Catalyst 3500 switches and all Catalyst 2900 XL switches with 8-MB DRAM support these QoS features. The Catalyst 2900 XL with 4-MB DRAM does not support QoS features
I have the same dilemma as you but I am reading that the 6509 with the PFC should trust dscp vs CoS. We have layer 3 6509's and layer 2 6509's but all have the pfc. I have not been able to validate it in a lab environment and would like the same clarification before I attempt it in production.
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