The QoS scheduler only has access to the length of the layer 3 packet. To allow the scheduler to be more accurate you can use the frame-overhead attribute to account for the layer 2 frame overhead.

policy

This keyword exists for 'organizational' purposes only, to structure the configuration path for relevant command strings.

qos

This keyword exists for 'organizational' purposes only, to structure the configuration path for relevant command strings.

name <policy-name>

Specifies the name of a QoS policy.

shaper

This keyword exists for 'organizational' purposes only, to structure the configuration path for relevant command strings.

frame-overhead <overhead>

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.