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considered non-pausable (“drop”) when priority-based flow control is enabled until no-drop is specifically
turned on.
9.2.1. PFC Operation and Behavior
PFC uses a new control packet defined in IEEE 802.1Qbb and therefore is not compatible with IEEE 802.3
Annex 31B flow control. An interface that is configured for PFC will be automatically disabled for flow
control. When PFC is disabled on an interface, the flow control configuration for the interface becomes active.
Any flow control frames received on a PFC configured interface are ignored.
Each priority is configured as either drop or no-drop. If a priority that is designated as no-drop is congested,
the priority is paused. Drop priorities do not participate in pause. You must configure the same no-drop
priorities across the network in order to ensure end-to-end lossless behavior.
Operator configuration of PFC is used only when the port is configured in a manual role. When
interoperating with other equipment in a manual role, the peer equipment must be configured with identical
PFC priorities and VLAN assignments. Interfaces not enabled for PFC ignore received PFC frames. Ports
configured in auto- upstream or auto-downstream roles receive their PFC configuration from the
configuration source and ignore any manually-configured information.
Note: This feature is configurable on physical full duplex interfaces only. To enable PFC on a Port-
channel interface, the member interfaces must have the same configuration.
When PFC is disabled, the interface defaults to the IEEE 802.3 flow control setting for the interface. PFC is
disabled by default.
If you enable priority-based flow control for a particular priority value on an interface, make sure 802.1p
priority values are mapped to CoS values (see “CoS”).
9.2.2. Configuring PFC
The network in this example handles standard data traffic and traffic that is time sensitive (such as voice and
video). The time-sensitive traffic requires a higher priority than standard data traffic. All time-sensitive traffic
is configured to use VLAN 100 and has an 802.1p priority of 5, which is mapped to hardware queue 4. The
hosts that frequently send and receive the
time-sensitive
traffic are connected to ports 3, 5, and 10, so PFC is
enabled on these ports with 802.1p priority 5 traffic as no-drop. The configuration also enables VLAN tagging
so that the 802.1p priority is identified. This example assumes that VLAN 100 has already been configured.
Caution! All ports may be briefly shutdown when modifying either flow control or PFC settings. PFC
uses a control packet defined in 802.1Qbb and is not compatible with 802.3x FC.
1. Map 802.1p priority 5 to traffic class 4. For more information about traffic classes, see “CoS”.
(QCT) #configure
(QCT) (Config)#queue cos-map all 5 4
2. Enter Interface Configuration mode for ports 3, 5, and 10.
(QCT) (Config)#interface range 0/3,0/5,0/10
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