pfifo_fast is the default qdisc of each interface.
Whenever an interface is created, the pfifo_fast qdisc is automatically used as a queue. If another qdisc is attached, it preempts the default pfifo_fast, which automatically returns to function when an existing qdisc is detached.
The algorithm is very similar to that of the classful tc-prio(8) qdisc. pfifo_fast is like three tc-pfifo(8) queues side by side, where packets can be enqueued in any of the three bands based on their Type of Service bits or assigned priority.
Not all three bands are dequeued simultaneously - as long as lower bands have traffic, higher bands are never dequeued. This can be used to prioritize interactive traffic or penalize 'lowest cost' traffic.
Alexey N. Kuznetsov,
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