Thank you for asking this question in the support community. The command qos trust cos is disabled by default in the NV1638 on all ports. Therefore, you will need to issue the command qos trust cos on every port where you desire the interface to trust cos if received 802.1P CoS values are considered valid (i.e., no need to reclassify) and do not need to be tagged with the default value. When set to untrusted, the default-cos value for the interface is used.
When mapping DSCP-COS, the switch will pass the original DSCP and mapped COS values on egress (to another switch). I hope that makes sense, but please do not hesitate to reply to this post with additional information or questions. I will be happy to help in any way I can.
That was clear, to confirm.
qos trust cos is not needed as DSCP will be mapped to COS and not be original COS from devices which would require qos trust cos
That leaves a question of should it be added for traffic on the (ingress)
Ingress traffic will pass original DSCP and mapped COS.
If qos trust cos is not added
Original DSCP will be mapped using DSCP-COS map
If qos trust cos is added
Mapped COS will be recognized
Both are the same QOS value, not sure if adding qos trust cos would be a benefit and decrease Adtran switch cpu by not requiring DSCP-COS map for ingress traffic.
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It depends on the network, and how much knowledge/control there is over the network. If the overall goal is the monitor DSCP traffic and change the Layer 2 CoS value, then the DSCP-COS map may be what you need. If you simply want to change the value of all CoS frames that arrive, then you can use the CoS queue. If the CoS values are already set as desired when arriving into the switch, then trust seems appropriate. These different functions serve various applications. Here is the Configuring Ethernet Switch QoS and CoS in AOS guide, which explains in detail the different CoS and QoS setting for AOS switches. Please, let me know what questions you have.
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