This articles describes Calculation of egress shaping overhead

When shaping-rate is used in the class-of-service configuration, egress shaping overhead calculation is differ depends on the interface type.

For the logical interface, the shaping-rate overhead is calculated based on Layer 2 size.

Example:

set class-of-service interfaces ge-0/0/0 unit 0 shaping-rate XXX

For the physical interface, the shaping-rate overhead is calculated based on Layer 1 size.

"overhead" option is available for Layer 2 overhead adjustment

Example:

set class-of-service interfaces ge-0/0/0 shaping-rate XXX

set class-of-service interfaces ge-0/0/0 overhead -20

* overhead option is only available for physical interface.

When overhead -20 is configured, the shaping-rate will be calculated based on Layer 2 frame size.

overhead -20 => 8 (preamble) + 12 (interframe-gap)