74 Flow control
Flow control
Feature overview IEEE 802.3x flow control is specified in IEEE 802.3x. Flow control allows traffic
from one device to be throttled for a specified period of time. It is defined for
devices that are directly connected. To inhibit transmission of data frames from
another device on the LAN, a device transmits a PAUSE frame, as defined in the
specification.
Flow control is configurable per port as enabled, disabled, or in auto mode. In
auto mode, the ability to send a PAUSE frame is determined based on the
autonegotiation resolution for the port (autonegotiation must also be enabled on
the port).
Since flow control is implemented only on full-duplex links, there is no
ambiguity as to the identity of the devices involved.
Operation When flow control is enabled and a valid PAUSE frame is received on a port, the
switch will not transmit frames sent by the switch fabric on that port for the
period of time specified in the PAUSE frame. When the specified time has
elapsed or a valid PAUSE frame is received on the port containing a zero pause
time, the switch again transmits traffic on the port.
The switch initiates the PAUSE operation when the fabric cannot keep up with
the amount of frames being switched; this is detected based on the amount of
memory used by packets on an input port. When the memory usage meets a
certain threshold, the switch transmits a PAUSE frame on the port. If the port
continues to receive packets after this operation is applied, the packets are
discarded. When the utilization returns to a specified low threshold, the switch
sends a PAUSE frame with a pause time of zero, indicating that the remote port
can transmit frames again.
Defaults By default, flow control is disabled on all ports.
Configuration
examples
The following example enables flow control on a port:
To Next Page
