Enterasys Networks 5H1XX - Manual (Page 137)

Class of Service C-1
C
Class of Service
Class of Service support on the switch allows you to assign mission-critical data to 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 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 (refer to
Appendix B ) 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
Section 7.10 .
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.