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The command is accepted on the ports and the port-channel interfaces. I'm guessing that the port-channel interface is key for aggregate interfaces. The switch ports see only LACP frames and I'm not sure whether any COS tags are present, so trusting COS would seem to be irrelevant. I don't have a firm answer (nor do I see this question answered in the documents Link Aggregation Control Protocol (LACP) in AOS or Configuring Ethernet Switch QoS and CoS in AOS). I don't think there's any danger in using the command for both the ports and port-channel. Perhaps someone will have a better answer.