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Chapter 9: Port Parameters
150
Enabling or Disabling Flow Control
When a port that is operating in full-duplex mode needs to temporarily stop
its local or remote counterpart from sending any further packets, it initiates
flow control by sending what are known as pause packets. Pause packets
instruct the link partner to stop sending packets to allow the sender of the
packets time to process the packets already stored in its buffers.
There are two aspects to flow control on the ports on the switch. The first
is whether or not a port will issue pause packets during periods of buffer
congestion. The other is whether or not a port will stop sending packets
when it receives pause packets from another network device. You can
control both of these aspects of flow control on the ports on the switch.
Flow control is set with the FLOWCONTROL RECEIVE command and the
the FLOWCONTROL SEND command. The formats of the commands
are:
flowcontrol send on|off
flowcontrol receive on|off
The FLOWCONTROL SEND command controls whether or not a port
sends pause packets during periods of packet congestion. If you set it to
ON, the port sends pause packets when it reaches the point of packet
congestion. If you set it to OFF, the port does not send pause packets. At
the default setting, the send portion of flow control is off.
The FLOWCONTROL RECEIVE command is used to control whether or
not a port stops transmitting packets when it receives pause packets from
its local or remote counterpart. If you set it to ON, a port stops transmitting
packets when it receives pause packets. If you set it to OFF, a port does
not stop transmitting packets when it receives pause packets. At the
default setting, the receive portion of flow control is off.
The commands are located in the Port Interface mode. This example
configures ports 12 and 13 to 100Mbps, full-duplex mode. The receive
portion of flow control is disabled so that the ports ignore any pause
packets that they receive from their link partners:
awplus> enable
awplus# configure terminal
awplus(config)# interface port1.0.12,port1.0.13
awplus(config-if)# speed 100
awplus(config-if)# duplex full
awplus(config-if)# flowcontrol receive off