HP A6600 Configuration Manual page 53

FIFO does not address congestion problems. If only one FIFO output/input queue exists on a port, you
cannot ensure timely delivery of mission-critical or delay-sensitive traffic or smooth traffic jitter. The situation
worsens if malicious traffic is present to occupy bandwidth aggressively. To control congestion and prioritize
forwarding of critical traffic, you must use other queue scheduling mechanisms, where multiple queues can
be configured. Within each queue, however, FIFO is still used.
By default, FIFO queuing is used on interfaces.
PQ
Figure 16 PQ
Packets to be sent
through this interface
PQ is designed for mission-critical applications. The key feature of mission-critical applications is that they
require preferential service to reduce the response delay when congestion occurs. Priority queuing can
flexibly determine the order of forwarding packets by network protocol (for example, IP and IPX), incoming
interface, packet length, source/destination address, and so on. Priority queuing classifies packets into four
queues: top, middle, normal, and bottom, in descending priority order. By default, packets are assigned to
the normal queue. Each of the four queues is a FIFO queue.
Priority queuing schedules the four queues strictly according to the descending order of priority. It sends
packets in the queue with the highest priority first. When the queue with the highest priority is empty, it sends
packets in the queue with the second highest priority. In this way, assign the mission-critical packets to the
high priority queue to ensure they are always served first. The common service packets are assigned to the
low-priority queues and transmitted when the high-priority queues are empty.
The disadvantage of priority queuing is that packets in the lower priority queues cannot be transmitted if
packets exist in the higher queues for a long time when congestion occurs.
High queue
Middle queue
Normal queue
Classify
Bottom queue
Packets sent
Schedule Sending queue
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Interface