Concept Guide

Using Ethernet Pause Frames for Flow Control
Ethernet pause frames and threshold settings are supported on the Dell EMC Networking OS.
Ethernet Pause Frames allow for a temporary stop in data transmission. A situation may arise where a sending device may transmit data
faster than a destination device can accept it. The destination sends a PAUSE frame back to the source, stopping the sender’s
transmission for a period of time.
An Ethernet interface starts to send pause frames to a sending device when the transmission rate of ingress trac exceeds the egress
port speed. The interface stops sending pause frames when the ingress rate falls to less than or equal to egress port speed.
The globally assigned 48-bit Multicast address 01-80-C2-00-00-01 is used to send and receive pause frames. To allow full-duplex ow
control, stations implementing the pause operation instruct the MAC to enable reception of frames with destination address equal to this
multicast address.
The PAUSE frame is dened by IEEE 802.3x and uses MAC Control frames to carry the PAUSE commands. Ethernet pause frames are
supported on full duplex only.
If a port is over-subscribed, Ethernet Pause Frame ow control does not ensure no-loss behavior.
Restriction: Ethernet Pause Frame ow control is not supported if PFC is enabled on an interface.
Control how the system responds to and generates 802.3x pause frames on Ethernet interfaces. The default is rx o tx o. INTERFACE
mode. flowcontrol rx [off | on] tx [off | on]| [monitor session-ID]
Where:
rx on: Processes the received ow control frames on this port.
rx off: Ignores the received ow control frames on this port.
tx on: Sends control frames from this port to the connected device when a higher rate of trac is received.
tx off: Flow control frames are not sent from this port to the connected device when a higher rate of trac is received.
monitor session-ID: Enables mirror ow control frames on this port.
Changes in the ow-control values may not be reected automatically in the show interface output. To display the change, apply the
new ow-control setting, shutdown the interface using the shutdown command, enable the interface using the no shutdown
command, and use the show interface command to verify the changes.
Enabling Pause Frames
Enable Ethernet pause frames ow control on all ports on a chassis or a line card. If not, the system may exhibit unpredictable behavior.
NOTE
: Changes in the ow-control values may not be reected automatically in the show interface output. As a
workaround, apply the new settings, execute shut then no shut on the interface, and then check the running-cong of the
port.
NOTE: If you disable rx flow control, Dell EMC Networking recommends rebooting the system.
The ow control sender and receiver must be on the same port-pipe. Flow control is not supported across dierent port-pipes.
To enable pause frames, use the following command.
• Control how the system responds to and generates 802.3x pause frames on the Ethernet ports.
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Interfaces