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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter8 Configuring Interface Characteristics
Configuring Layer 2 Interfaces
Configuring IEEE 802.3X Flow Control
Flow control enables connected Ethernet ports to control traffic rates during congestion by allowing
congested nodes to pause link operation at the other end. If one port experience s con gestion 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 devi ce of the
congestion 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 t he c ongest ion pe rio d.
Note You must not configure both IEEE 802.3X flowcontrol and quality of service (QoS) on a switch.
Before configuring flowcontrol on an interface, use the no mls qos global configuration c om mand
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 Gigabit Ethernet ports is receive off and send desired.
The default state for Fast Ethernet ports is receive off and send off.
Note On Catalyst 3550 switches, Gigabit Ethernet ports are ca pabl e o f r ece ivi ng and sen ding pa use
frames; Fast Ethernet ports can only receive pause frame s. The re fore, f or Fa st E th erne t p o rts, on ly
the conditions described with send off are applicable.
These rules apply to flow control settings on the device:
•receive on (or desired) and send on: Flow control operates in both directions; b oth the loc al 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 sen d pau se
frames if the attached device supports f lo w c ont r ol.
•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 sen d pa use fr am es ; t he p ort ca n rece iv e p au se fr am es.
•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 r ece ive d b y ei the r device .
Note For details on the command settings and the resulting flow control resolution on local and remote
ports, refer to the flowcontrol interface configuration command in the Catalyst 3550 Multilayer
Switch Command Reference for this release.