Defines an additional allowance to the length of a packet in bytes that a shaper can make during calculation of bandwidth. You can adjust this to make the QoS scheduler more accurate.
The QoS scheduler only knows the size of the L3 packet. The size of the L2 frame header can vary in size depending upon whether or not a VLAN header is present. By default the QoS scheduler allows for a 24 bytes L2 frame header. If the L2 frame header is bigger than 24 bytes then the QoS scheduler may send more traffic than the maximum bandwidth permits.