Configuring Ieee 802.3X Flow Control On Gigabit Ethernet Ports - Cisco Catalyst 2950 Software Configuration Manual

Configuring the Port Speed and Duplex Mode

Configuring IEEE 802.3X Flow Control on Gigabit Ethernet Ports

Flow control is supported only on 10/100/1000 ports and GBIC-module ports. Flow control enables
connected Gigabit Ethernet ports to control traffic rates during congestion by allowing congested nodes
to pause link operation at the other end. If one port experiences congestion and cannot receive any more
traffic, it notifies the other port to stop sending until the condition clears. When the local device detects
any congestion at its end, it can notify the link partner or the remote device by sending a pause frame.
Upon receipt of a pause frame, the remote device stops sending any data packets, which prevents any
loss of data packets during the congestion period.
Note We strongly recommend that you do not configure IEEE 802.3X flowcontrol when quality of service
(QoS) is configured on the switch. Before configuring flowcontrol on an interface, make sure to disable
QoS on the switch.
Flow control can be implemented in two forms, symmetric and asymmetric. The symmetric
implementation is suitable for point-to-point links, and asymmetric is suitable for hub-to-end node
connections, where it is desirable for the hub to pause the end system, but not vice-versa. You use the
flowcontrol interface configuration command to set the interface's ability to receive and send pause
frames to on, off, or desired. The default state for 10/100/1000 and GBIC-module ports is receive off
and send desired.
These rules apply to flow control settings on the device:
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For details on the command settings and the resulting flow control resolution on local and remote ports,
Note
refer to the flowcontrol interface configuration command in the Catalyst 2950 Desktop Switch
Command Reference for this release.
Catalyst 2950 Desktop Switch Software Configuration Guide
9-14
0 input errors, 0 CRC, 0 frame, 0 overrun, 0 ignored
0 input packets with dribble condition detected
0 packets output, 0 bytes, 0 underruns
0 output errors, 0 collisions, 2 interface resets
0 babbles, 0 late collision, 0 deferred
0 lost carrier, 0 no carrier
0 output buffer failures, 0 output buffers swapped out
receive on (or desired) and send on: Flow control operates in both directions; both the local and the
remote devices can send pause frames to show link congestion.
receive on (or desired) and send desired: The port can receive pause frames and can send pause
frames if the attached device supports flow control.
receive on (or desired) and send off: The port cannot send pause frames but can operate with an
attached device that is required to or can send pause frames; the port can receive pause frames.
receive off and send on: The port sends pause frames if the remote device supports flow control but
cannot receive pause frames from the remote device.
receive off and send desired: The port cannot receive pause frames but can send pause frames if the
attached device supports flow control.
receive off and send off: Flow control does not operate in either direction. In case of congestion, no
indication is given to the link partner, and no pause frames are sent or received by either device.
Chapter 9 Configuring Interface Characteristics
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