Enterasys Networks VH-8G-L3 - Manual (Page 38)

28 9033705-01
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.