Hi There,
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.
http://www.cisco.com/en/US/partner/docs/switches/datacenter/nexus5500/sw/qos/602_N1_1/b_5500_QoS_Config_602N11_chapter_0110.html
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.
Trust Boundaries
The trust boundary is enforced by the incoming interface as follows:
- All Fibre Channel and virtual Fibre Channel interfaces are automatically classified into the FCoE system class.
- By default, all Ethernet interfaces are trusted interfaces.The 802.1p CoS and DSCP are preserved unless the marking is configured. There is no default CoS to queue and DSCP to queue mapping. You can define and apply a policy to create these mappings. By default, without a user defined policy, all traffic is assigned to the default queue.
- Any packet that is not tagged with an 802.1p CoS value is classified into the default drop system class. If the untagged packet is sent over a trunk, it is tagged with the default untagged CoS value, which is zero.
- You can override the default untagged CoS value for an Ethernet interface or port channel.
- You can override the default untagged CoS value for an Ethernet interface or a port channel interface using the untagged cos cos-value command.
- You can override the default untagged Cos value for an Ethernet or a Layer 3 interface or a port channel interface using the untagged cos cos-value command.
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.
