9032579-03 Class Of Service 89
APPENDIX E. CLASS OF SERVICE
Class of Service support on the ELS100-24TX allows you to assign
mission-critical data a higher priority through the switch by delaying less
critical traffic during periods of congestion. Higher priority traffic through
the switch is serviced first before lower priority traffic. The Class of
Service capability of the ELS100-24 TX switch is imp lemented by a priority
queuing mechanism. Class of Service is b as ed on t he IEEE 802.1p draft
standard specification and allows you to define two priorities of traffic on
each switch port:
•high
•normal
As traffic enters the switch, it is assigned to one of the two priority levels
according to information located in the 802.1Q header tag of the frame
(see Appendix D, “Vir tual LANs”) or accord i ng to the incoming por t
number. Frames are then placed into one of two transmit queues on the
outbound switch port based on their priority level. Frames on the high
priority queue are transmitted first; w hen that queue e mpties, traffic on the
normal priority queue is transmitted. Whe n priority qu euing is being used,
each frame that passes through the switch contains a priority level in its
header tag. The priority infor mation m ay alr eady ex ist in incoming frames,
or be assigned by the swi tch. The determinati on of individual fram e
priority 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 default
priority level based on their incoming port (range: 0-7). The
assignment of priority per port is done via management using the
Port Priority Default parameter.
3. Priority levels of frames are compar ed against a reconfigured global
priority threshold setting, configured via management using the
Priority Threshold parameter. Those frames with levels equal to or
above the threshold are designat ed hi gh prio rity traffic ; those fram es
with levels below the t hreshold are designat ed normal priority tra ffic.
The default setting for the threshol d parameter is : 4 and above = H igh
Priority, 3 and below = Normal Priority.
Properly configured, the Class of Servi ce m ech anism assur es that d uring
congestion, the highest priority data does not get delayed by normal
priority traffic. The tagged header in the frame governs individual frame
priority.