During a test, we found the N5K attached FEX will override all the DSCP marking to 0 for the class-default traffic. Based on my understanding, the N5K will enable the qos trust on all access port by defual. But how about I configure a policy-map on the interface for DSCP marking, and want to leave the unclassified traffic as the original DSCP tag, is that possible?
Take a look at the QoS config guide, it has some of the answers you are after.
You should refer to the guide relevant to your OS version.
Any incoming packet not tagged with an 802.1p CoS value is assigned the default untagged CoS value of zero (which maps to the default Ethernet drop system class). You can override the default untagged CoS value for an Ethernet or EtherChannel interface.
On a Cisco Nexus device, you can configure a type qos policy map and untagged CoS on the same interface.
The trust boundary is enforced by the incoming interface as follows:
After the system applies the untagged CoS value, QoS functions the same as for a packet that entered the system tagged with the CoS value.
Hope that helps.
Thanks for your info. I read the document, what troubles me is that I need to keep the DSCP value, I didn't look into the CoS field, but I think what Nexus does on the L2 QoS value should not have impact on the L3 QoS value.
recently, I recreated the testing scenario on another N5K, it seems the DSCP value will remain when there is no QoS config under the interface. I don't have enough access to a testbed now, so I was wondering to get a quick answer here.