Appendix B. Flow Control - Enterasys Vertical Horizon VH-2402-L3 User Manual

Appendix B. Flow Control

Flow control is a mechanism which 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
which 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-2402-L3 switch 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.
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