3Com 7700 Configuration Manual page 204

196 C 7: Q S/ACL O
HAPTER O PERATION
Figure 2 SP
Packets sent through
this interface
The SP is designed for the key service application. A significant feature of the key
service is required for priority to enjoy the service to reduce the responding delay
when congestion occurs. Take 4 egress queues for each port as example, SP
divides the queue of port into up to 4 kinds, high-priority, medium-priority,
normal-priority and low-priority queues (which are shown as the Queue 3, 2, 1
and 0 in turn) with sequentially reduced priority.
During the progress of queue dispatching, the SP strictly follows the priority order
from high to low and gives preference to and sends the packets in the
higher-priority queue first. When the higher-priority queue is empty, the SP sends
the packets in the lower-priority groups. In this way, the SP can guarantee that key
service packets of higher priority are transmitted first, while the packets of lower
service priority are transmitted during the idling gap between higher priority
However, when congestion occurs and many packets are queued in the
higher-priority queue, messages in the lower-priority queue are set aside without
service until all high-priority messages are transmitted.
Traffic Mirroring
The traffic mirroring function copies the specified data packets to the monitoring
port for network diagnosis and troubleshooting.
Traffic Counting
With flow-based traffic counting, you can request a traffic count to count and
analyze the packets.
RED
When the congestion reaches a certain degree, the Switch 7700 selects some
frames to drop, using the RED algorithm. The RED alogrithm can alleviate the
excessive congestion. In addition, the global TCP synchronization caused by the
Tail-Drop algorithm can be avoided.
In the RED algorithm, every queue has a pair of high and low limits. This algorithm
also regulates that:
If the queue length is smaller than the low limit, no packet is discarded.
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If the queue length is greater than the high limit, all the packets that arrive
■
after the limit is reached are discarded.
If the queue length is between the high and low limits, the packets are
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discarded randomly as they arrive. Every new packet is given a random number.
high queue
middle queue
normal queue
Classify
bottom queue
Packets sent
Sending queue
Dequeue