3Com 10014298 manual Traffic Mirroring, Traffic Counting

160CHAPTER 7: QOS/ OPERATION

Figure 39 SP

Packets sent through this interface

Classify

high queue

Packets sent middle queue

normal queue

Sending queue

bottom queue Dequeue

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 response delay when congestion occurs. Take 4 egress queues for each port as example, SP divides the queue of a port into 4 kinds at most, 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, SP strictly follows the priority order from high to low and gives preference, and sends the packets in the higher-priority queue first. When the higher-priority queue is empty, SP sends the packets in the lower-priority groups. In this way, 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

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 7750 selects some frames to drop using the RED algorithm. The RED algorithm can alleviate the excessive congestion. Also, 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 packets are discarded.

■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 discarded randomly as they arrive. Every new packet is given a random number.