Enterasys Networks VH-8G-L3 manual Appendix B. Flow Control

Appendix B. Flow Control

Flow control is a mechanism that allows you to protect the switch from overload conditions and to keep additional traffic off the network when excessive congestion will result.

Flow Control for Half- and Full-Duplex Applications

Each port of the switch has a transmit queue which buffers frames to be sent out on that port. In this example, large amounts of data are being sent from Workstation A (connected to port X) and other ports on the switch to Workstation B (connected to port Y). The queue on port Y starts filling up with data until it reaches a determined threshold. The packet that causes the threshold to be exceeded triggers the flow-control function on the port from which the packet entered the switch, in this case port X.

Since port X is configured with flow control set to Enabled, the switch responds by initiating a pause frame (full-duplex applications) or back-pressure mechanism (half-duplex applications). The pause frame causes Workstation A to stop sending packets. After a certain amount of time has elapsed, determined by a value in the pause frame, Workstation A will resume sending data. Similarly, the back-pressure mechanism forces Workstation A to stop sending packets by inducing collisions on port X.

The pause-frame flow-control mechanism supported by the VH-8G-L3 conforms with the IEEE 802.3x specification for full-duplex flow control.

For the full-duplex pause-frame mechanism to work, the device connected to the switch port must also support IEEE 802.3x flow control.

Flow control is configurable per port in the Configure Ports screen of the console menu, using the on-board Web agent, or via SNMP.

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