Priority
The 3-bit priority field in the Q-tag provides eight priority levels. This is supported by
bridges in one standard way, with each vendor free to add additional capabilities. So the
default priority can be 0, but not the lowest possible priority, the order of importance of
frames is, from most important to least important, 7, 6, 5, 4, 3, 0, 2, 1.
In all implementations, each output bridge port has some number of output queues
from 1 to 8. The 3-bit priority field is used to decide in which queue to place each
frame for subsequent output. A bridge may implement any number from 1 to 8 queues,
and this may be a configurable parameter. Every bridge must offer the capability of
draining those queues in strict priority order. That is, if any frames are in the queue
to which priority 7 frames are assigned, then that queue must be selected for output.
If that queue is empty, then a frame is transmitted from the queue to which priority
6 frames are assigned, and so on through priorities 5, 4, 3, 0, 2, and finally 1. Which
priority values are mapped to which queues if fewer than 8 queues are implemented
are defined in IEEE Std. 802.
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