Enterasys Networks VH-8TX1MF Manuel d'utilisateur télécharger pdf (Page 45)

9033718 Flow Control 31

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. Figure B-1 illustrates flow control for both

half- and full-duplex applications.

Figure B-1. 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-8TX1UM/VH-8TX1MF 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.

Flow control is configurable per port in the Port Configuration Menu of the

console menus, using the on-board Web agent, or via SNMP.

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Enterasys Networks VH-8TX1MF Manuel d'utilisateur Page 45