Information About Flow Control; Link-Level Flow Control; Priority Flow Control - Cisco Nexus 5000 NX-OS Service Configuration Manual

Information About Flow Control

Information About Flow Control

Link-Level Flow Control

IEEE 802.3x link-level flow control allows a congested receiver to communicate a transmitter at the other
end of the link to pause its data transmission for a short period of time. The link-level flow control feature
applies to all the traffic on the link.
The transmit and receive directions are separately configurable. By default, link-level flow control is disabled
for both directions.
On the Cisco Nexus device, Ethernet interfaces do not automatically detect the link-level flow control capability.
You must configure the capability explicitly on the Ethernet interfaces.
On each Ethernet interface, the switch can enable either priority flow control or link-level flow control (but
not both).

Priority Flow Control

Priority flow control (PFC) allows you to apply pause functionality to specific classes of traffic on a link
instead of all the traffic on the link. PFC applies pause functionality based on the IEEE 802.1p CoS value.
When the switch enables PFC, it communicates to the adapter which CoS values to apply the pause.
Ethernet interfaces use PFC to provide lossless service to no-drop system classes. PFC implements pause
frames on a per-class basis and uses the IEEE 802.1p CoS value to identify the classes that require lossless
service.
In the switch, each system class has an associated IEEE 802.1p CoS value that is assigned by default or
configured on the system class. If you enable PFC, the switch sends the no-drop CoS values to the adapter,
which then applies PFC to these CoS values.
The default CoS value for the FCoE system class is 3. This value is configurable.
By default, the switch negotiates to enable the PFC capability. If the negotiation succeeds, PFC is enabled
and link-level flow control remains disabled regardless of its configuration settings. If the PFC negotiation
fails, you can either force PFC to be enabled on the interface or you can enable IEEE 802.x link-level flow
control.
If you do not enable PFC on an interface, you can enable IEEE 802.3X link-level pause.
Ensure that pause no-drop is configured on a class map for link-level pause.
Note
By default, link-level pause is disabled.
Cisco Nexus 5000 NX-OS Quality of Service Configuration Guide, Release 5.1(3)N2(1)
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Configuring Queuing and Flow Control
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