Configuring Priority Flow Control; About Priority Flow Control - Cisco Nexus 9000 Series Configuration Manual

Configuring Priority Flow Control

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About Priority Flow Control

Priority flow control (PFC; IEEE 802.1Qbb), which is also referred to as Class-based Flow Control (CBFC)
or Per Priority Pause (PPP), is a mechanism that prevents frame loss that is due to congestion. PFC is similar
to 802.3x Flow Control (pause frames) or link-level flow control (LFC). However, PFC functions on a per
class-of-service (CoS) basis.
When a buffer threshold is exceeded due to congestion, LFC sends a pause frame to its peer to pause all data
transmission on the link for a specified period of time. When the congestion is mitigated (traffic comes under
the configured threshold), a resume frame is generated to restart data transmission on the link.
In contrast, during congestion, PFC sends a pause frame that indicates which CoS value needs to be paused.
A PFC pause frame contains a 2-octet timer value for each CoS that indicates the length of time that the traffic
needs to be paused. The unit of time for the timer is specified in pause quanta. A quanta is the time that is
required for transmitting 512 bits at the speed of the port. The range is from 0 to 65535. A pause frame with
a pause quanta of 0 indicates a resume frame to restart the paused traffic.
Note Only certain classes of service of traffic can be flow controlled while other classes are allowed to operate
normally.
About Priority Flow Control, on page 149
Prerequisites for Priority Flow Control, on page 150
Guidelines and Limitations for Priority Flow Control, on page 150
Default Settings for Priority Flow Control, on page 153
Configuring Priority Flow Control, on page 153
Enabling Priority Flow Control on a Traffic Class, on page 155
Configuring a Priority Flow Control Watchdog Interval, on page 158
Configuring Pause Buffer Thresholds and Queue Limit Using Ingress Queuing Policy, on page 161
Configuring QoS Pause Buffer and Dynamic Queue Limits, on page 163
Verifying the Priority Flow Control Configuration, on page 164
Configuration Examples for Priority Flow Control, on page 164
Cisco Nexus 9000 Series NX-OS Quality of Service Configuration Guide, Release 7.x
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