HP A6600 Configuration Manual page 76

SP queuing
SP queuing is designed for mission-critical applications, which require preferential service to reduce the
response delay when congestion occurs.
Figure 24 SP queuing schematic diagram
Packets to be sent through
this port
As shown in Figure
descending priority order.
SP queuing schedules the eight queues 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, and so on. Assign mission-critical packets to the high-priority queue
to ensure they are always served first and common service packets to the low-priority queues and transmitted
when the high-priority queues are empty.
The disadvantage of SP queuing is packets in the lower-priority queues cannot be transmitted if packets exist
in the higher-priority queues. This may cause lower-priority traffic to starve to death.
SP queuing involves:
 Basic SP queuing—Contains multiple queues, with each queue corresponding to a different priority.
These queues are scheduled in descending order of priority.
Multimode SP queuing—Extends basic SP queuing by providing multiple queue scheduling modes.

Multimode SP queuing operates in one of the following modes:
SP mode 0—In this mode, basic SP queuing is used.

SP mode 1—In this mode, when the remaining external memory space is sufficient, basic SP queuing

is used. When the remaining external memory space is 0, the scheduling algorithm can preferentially
forward lower-priority packets stored in the internal memory of the chip, even if higher priority packets
are waiting to be scheduled in the external memory.
SP mode 2—In this mode, packets stored in the internal memory of the chip are forwarded

preferentially. If no packets are stored in the internal memory of the chip, all the packets are scheduled
using the basic SP queuing. The disadvantage of SP mode 2 is the bus bandwidth of the external
memory is decreased.
Queue 7
Queue 6
......
Queue 1
Packet
classification
Queue 0
24, SP queuing classifies eight queues on a port into eight classes, numbered 7 to 0 in
High priority
Sent packets
Interface
Sending queue
Queue
scheduling
Low priority
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