Cabletron Systems ELS100-24TXG Installation And User Manual

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Cabletron Systems ELS100-24TXG Installation And User Manual Manual pdf 103 page image

9032947 Class Of Service 89APPENDIX E. CLASS OF SERVICEClass of Service support on the ELS100-24TXG allows you to assignmission-critical data a higher priority through the switch by delaying lesscritical traffic during periods of congestion. Higher priority traffic throughthe switch is serviced first before lower priority traffic. The Class ofService capability of the ELS100-24TXG switch is implemented by apriority queuing mechanism. Class of Service is based on the IEEE802.1p draft standard specification and allows you to define two prioritiesof traffic on each switch port:• high• normalAs traffic enters the switch, it is assigned to one of the two priority levelsaccording to information located in the 802.1Q header tag of the frame(see Appendix D, “Virtual LANs”) or according to the incoming portnumber. Frames are then placed into one of two transmit queues on theoutbound switch port based on their priority level. Frames on the highpriority queue are transmitted first; when that queue empties, traffic on thenormal priority queue is transmitted. When priority queuing is being used,each frame that passes through the switch contains a priority level in itsheader tag. The priority information may already exist in incoming frames,or be assigned by the switch. The determination of individual framepriority is based on the following rules:1. Incoming tagged frames contain a priority level (range: 0-7)2. Incoming non-tagged frames are assigned a preconfigured defaultpriority level based on their incoming port (range: 0-7). Theassignment of priority per port is done via management using thePort Priority Default parameter.3. Priority levels of frames are compared against a reconfigured globalpriority threshold setting, configured via management using thePriority Threshold parameter. Those frames with levels equal to orabove the threshold are designated high priority traffic; those frameswith levels below the threshold are designated normal priority traffic.The default setting for the threshold parameter is: 4 and above = HighPriority, 3 and below = Normal Priority.Properly configured, the Class of Service mechanism assures that duringcongestion, the highest priority data does not get delayed by normalpriority traffic. The tagged header in the frame governs individual framepriority.