Link-Level Flow Control; Priority Flow Control - Cisco Nexus 5000 Series Configuration Manual

Link-Level 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 5000 Series switch, 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.
Cisco Nexus 5000 Series NX-OS Quality of Service Configuration Guide
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◦Class of Service—Matches traffic based on the CoS field in the frame header.
◦DSCP—Classifies traffic based on the Differentiated Services Code Point (DSCP) value in
the DiffServ field of the IP header.
◦IP Real Time Protocol—Classifies traffic on the port numbers used by real-time applications.
◦Precedence—Classifies traffic based on the precedence value in the type of service (ToS)
field of the IP header.
◦Protocol—Classifies traffic based on the protocol field of the IP header.
◦Policy—The actions that are performed on the matching traffic are as follows:
Note This policy can be attached to the system or to any interface. It applies to input traffic
only.
◦QoS Group—Sets the qos-group corresponding to the system class this traffic flow is mapped
to.
Configuring QoS
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