Deployment Guide

Using Ethernet Pause Frames for Flow Control
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 traffic 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 flow 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 defined 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 flow control does not ensure no-loss behavior.
Restriction: Ethernet Pause Frame flow 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 off tx off.
INTERFACE mode. flowcontrol rx [off | on] tx [off | on] monitor session-ID
Where:
rx on: Processes the received flow control frames on this port.
rx off: Ignores the received flow control frames on this port.
tx on: Sends control frames from this port to the connected device when a higher rate of traffic is received.
tx off: Flow control frames are not sent from this port to the connected device when a higher rate of traffic is received.
monitor session-ID: Enables mirror flow control frames on this port.
Changes in the flow-control values may not be reflected automatically in show interface output. To display the change, apply
the new flow-control setting, perform a shutdown followed by a no shutdown on the interface, and then check re-display the
show interface output for the port.
Threshold Settings
When the transmission pause is set (tx on), you can set three thresholds to define the controls more closely. Ethernet pause
frames flow control can be triggered when either the flow control buffer threshold or flow control packet pointer threshold is
reached.
The following thresholds are provided:
● Number of flow-control packet pointers: from 1 to 2047 (default = 75)
● Flow-control buffer threshold in KB: from 1 to 2013 (default = 49KB)
● Flow-control discard threshold in KB: from 1-2013 (default= 75KB)
The pause is started when either the packet pointer or the buffer threshold is met (whichever is met first). When the discard
threshold is met, packets are dropped.
The pause ends when both the packet pointer and the buffer threshold fall below 50% of the threshold settings.
The discard threshold defines when the interface starts dropping the packet on the interface. This may be necessary when a
connected device doesn’t honor the flow control frame sent by the switch.
The discard threshold should be larger than the buffer threshold so that the buffer holds at least hold at least three packets.
Interfaces
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