7 Frame Preemption

Frame Preemption is defined in the IEEE 802.1Qbu and IEEE 802.3br standards.

Frame Preemption is the ability to suspend the transmission of a non-time-critical frame, and allow for one or more time-critical frames to be transmitted. When the time-critical frames have been transmitted, the transmission of the non-time-critical frame is resumed. A non-time-critical frame can be preempted multiple times.

The use of Frame Preemption with Time Aware Shaping reduces the guard band needed from the size of the largest possible interfering frame to the size of the largest possible interfering fragment, as shown in the following figure.

Figure 7-1. Time Aware Shaping without Frame Preemption
A diagram of a diagram Description automatically generated
Figure 7-2. Time Aware Shaping with Frame Preemption
Diagram of a diagram Description automatically generated

Frame Preemption must be enabled on both the egress port level and on the egress port/queue level.

Frame Preemption is disabled by default on the port level and disabled by default on the port/queue level.