Operation Manual – QoSH3C S7500 Series Ethernet Switches Chapter 1 QoS Configuration1-11In the RED algorithm, an upper limit and a lower limit are set for each queue, and it isstipulated that:z When the queue length is smaller than the lower limit, no packet is dropped.z When the queue length is bigger than the upper limit, all received packets alldropped.z When the queue length is in the range of the upper limit and the lower limit, thereceived packets are dropped at random. In this case, a number is assigned toeach received packet and then compared with the drop probability of the currentqueue. If the assigned number is bigger than the drop probability, the receivedpacket is dropped. The longer a queue is, the higher the drop probability is.However, there is a maximum drop probability.In the RED algorithm, packets are dropped at random so that global TCPsynchronization is avoided. When packets in a TCP connection are dropped and sentat a low rate, packets in other TCP connections are still sent at a high rate. In this way,packets in a part of TCP connections are sent at a high rate in any case. Thus, theutilization rate of bandwidth is improved.1.2 QoS Supported by the S7500 Series Ethernet SwitchesTable 1-4 QoS features supported by the S7500 series Ethernet switches and relatedcommandsQoS Specification Related commandPriority mappingSupport only the802.1p-priority-to-local-queuemappingqos cos-local-precedence-mapPort priority Supported priority priority-levelPriority according to whicha packet is put into anoutput queueSupported priority-trustTP Supported traffic-limitPriority marking Supported traffic-priorityRedirect Supported traffic-redirectQueue scheduling Support SP, RRand WRR queue-schedulerRate limiting Supported line-rateGuaranteed bandwidth Supported traffic-bandwidthCongestion avoidance RED operation traffic-redTraffic statistics Supported traffic-statisticInbound CAR Supported inboundcar { enable | disable }
