Enterasys Networks VH-8G manual Appendix C. Class of Service

APPENDIX C. CLASS OF SERVICE

Class of Service support on the VH-8G switch 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-8G 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 44.

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.

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