Frame Preemption functional description (using ethtool and tc)

Frame preemption is defined in IEEE 802.1Qbu and 802.3br and allows low priority frames to be suspended by time-critical frames of higher priority.

This section describes how to enable frame preeemption using the tools ethtool and tc. This is the standard way of enabling the frame preemption.

This is working fine only for the following platforms:

To enable the frame preemption the port the following command can be run

To see if the frame preemption is enabled, the following command can be used:

Where the mm stands for MAC-merge

More information regarding ethtool can be found here

Now that the frame preemption is enabled, we need to decide which frames can be preempted. To do that we can use the tc command to set which queues contain preemptable traffic.

For example the frames in queue 2 to be preemptable the following command can be used:

Where:

More information regarding the tc command can be found here

Before frames can be preempted, they must be assigned to the queues for which frame preemption is enabled.

A natural way to do this is the pcp value of VLAN tagged frames. It is important to enable trust for pcp on the ingress ports, otherwise this value is ignored in the queue classification.