QoS: Ethernet Class of Service

Ethernet supports a 3 bit QoS marking field, but it only exists when using 802.1Q or ISL headers.  802.1Q defines its QoS field as the 3 most significant bits of the 2 byte Tag Control field calling this the user-priority bits.  ISL defines the 3 least significant bits from the 1 byte User field calling it the Class of Service (CoS).  Both are usually referred to as CoS regardless of trunking type.

L2QoS.PNG

 

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