About Priority Flow Control
Priority-based flow control (PFC) pause frames are used in lossless ethernet to pause the link partner from sending packets. These PFC pause frames can propagate throughout the entire network and can cause the traffic on the PFC streams to halt. The PFC watchdog is a mechanism designed to detect and resolve any PFC storms (queue-stuck condition) in the network. You can configure a PFC watchdog interval to detect whether packets in a no-drop queue are drained within a specified time period. When this time period is exceeded, all outgoing packets are dropped on interfaces that match the PFC queue that is not being drained.
This process involves monitoring PFC reception on each port and identifying ports that are receiving an unusual number of sustained pause frames. Once detected, the watchdog module can enforce several actions on these ports, which include generating a syslog message for network management systems, shutting down the queue, and automatically restoring the queue (after the PFC storm stops).
The PFC watchdog feature has the following capabilities:
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It monitors the PFC-enabled queues to identify the reception of an unusually high number of PFC pause frames in a given interval (Watchdog interval.)
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It notifies the watchdog module when an excessive number PFC frames are received and traffic on the corresponding queues is halted for a time interval.
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Upon receiving such notifications, the Watchdog module initiates the shutdown timer and changes the queue state to a wait-to-shutdown state.
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It checks for PFC frames and if the traffic in the queue is stuck at regular intervals during the shutdown interval. If the traffic is not stuck because the queue didn't receive any PFC frames, the queue reverts to the monitored state.
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If the traffic is stuck for an extended period and the shutdown-timer expires, the queue transitions to a drop state and the PFC watchdog begins to drop all packets.
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It checks the queue for PFC frames and whether the traffic in the queue is still stuck at regular intervals. If traffic is stuck in the queue as PFC packets continue to arrive, the queue stays in the drop or shutdown state.
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When the traffic is no longer stuck, the auto-restore timer starts. At regular intervals, the module checks if the queue is stuck because of PFC frames.
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If the queue receives PFC frames during the last auto-restore interval, the auto-restore timer is reset upon expiration.
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If the queue receives no PFC frames during the last auto-restore interval, the watchdog module restores the queue, and traffic resumes.
Note |
The PFC watchdog feature is supported only for no-drop queue. |