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
In this sense this qdisc is magic, and unlike other qdiscs.
The algorithm is very similar to that of the classful
is like three
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.
Each band can be txqueuelen packets long, as configured with
Additional packets coming in are not enqueued but are instead dropped.
for complete details on how TOS bits are translated into bands.
The length of the three bands depends on the interface txqueuelen, as
Does not maintain statistics and does not show up in tc qdisc ls. This is because
it is the automatic default in the absence of a configured qdisc.