Cabletron Systems ELS100-24TXG - Manual (Page 103)

9032947 Class Of Service 89
APPENDIX E. CLASS OF SERVICE
Class of Service support on the ELS100-24TXG 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 ELS100-24TXG switch is implemented by a
priority queuing mechanism. Class of Service is based on the 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, “Virtual LANs”) or according to the incoming port
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; when that queue empties, traffic on the
normal priority queue is transmitted. When priority queuing is being used,
each frame that passes through the switch contains a priority level in its
header tag. The priority information may already exist in incoming frames,
or be assigned by the switch. The determination of individual frame
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 compared 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 designated high priority traffic; those frames
with levels below the threshold are designated normal priority traffic.
The default setting for the threshold parameter is: 4 and above = High
Priority, 3 and below = Normal Priority.
Properly configured, the Class of Service mechanism assures that during
congestion, the highest priority data does not get delayed by normal
priority traffic. The tagged header in the frame governs individual frame
priority.