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9033645-01 Class of Service 95
APPENDIX C. CLASS OF SERVICE
Class of Service support on the VH-2402S 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 VH-2402S switch is implemented by a priority queuing
mechanism. Class of Service is based on the IEEE 802.1p 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 B, “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 console interface
or via SNMP. See “Configuring Traffic Classes” on page 46.
3. Priority levels of frames are compared against a preconfigured global
priority threshold setting. 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.