Priority Flow Control (PFC) is a mechanism used in data center networks to ensure lossless transmission for high-priority traffic by pausing traffic when congestion is detected. While PFC helps in managing traffic congestion, it can potentially lead to a situation known as a PFC deadlock. To address this issue, network devices employ a PFC watchdog mechanism to detect and mitigate PFC deadlocks.
Understanding PFC Deadlock
A PFC deadlock occurs when multiple devices in a network are continuously sending PFC pause frames to each other, leading to a situation where traffic is indefinitely paused, causing a complete halt in data transmission. This deadlock can severely impact network performance and application availability.
PFC Watchdog Mechanism
The PFC watchdog is designed to detect and resolve PFC deadlocks. It monitors the duration of PFC pause frames and takes corrective actions if a potential deadlock is detected.
Key Components and Functions of PFC Watchdog
Monitoring PFC Pause Frames: The PFC watchdog continuously monitors the network for PFC pause frames. It keeps track of the duration for which these frames are active for each priority class on each port.
The following command can be used to enable PFC watchdog functionality:
set class-of-service interface <interface-name> pfc-watchdog code-point <cos> enable <true | false>
Timeout Threshold: A configurable timeout threshold is set for PFC pause frames. If a pause frame for a specific priority exceeds this threshold, it indicates a potential deadlock situation.
The detect timer can be configured by the following commands:
set class-of-service pfc-watchdog granularity <10 | 100>
set class-of-service pfc-watchdog code-point <cos> detect-interval <detect-interval>
Deadlock Detection: When the duration of a PFC pause frame surpasses the timeout threshold, the PFC watchdog triggers a deadlock detection process. This process identifies the ports and priority classes involved in the deadlock.
Restore Actions: Once a potential deadlock is detected, the PFC watchdog takes restore actions to break the deadlock. These actions typically include:
Forward: During the PFC deadlock recovery process, the received PFC PAUSE frame will be ignored, and the internal scheduler will resume forwarding the traffic.
Drop: drops received data packets.
The restore action can be configured by the following commands:
set class-of-service pfc-watchdog restore-action <forward | drop>
Re-enabling PFC: After a predefined period, the PFC watchdog re-enables PFC for the affected priority class. This period allows the network to stabilize and clear any residual congestion that could cause another deadlock.
The restore time can be configured by the following commands:
set class-of-service pfc-watchdog code-point <cos> restore-interval <restore-interval>
Restrictions and Guidelines
When you configure PFC watchdog, follow these restrictions and guidelines:
PFC should be enabled on the interface before enabling PFC watchdog.
PFC watchdog is only supported on Trident3 and Tomahawk3 platforms.
Configuring PFC Watchdog
Procedure
Step 1 Enable PFC on the interface before enabling PFC watchdog.
set class-of-service pfc-profile <pfc-profile-name> [code-point <cos> drop <true | false>]
set class-of-service interface <interface-name> pfc-profile <pfc-profile-name>
Step 2 Enable PFC watchdog. By default, PFC watchdog is disabled.
set class-of-service interface <interface-name> pfc-watchdog code-point <cos> enable <true | false>
Step 3 Configure the time interval of PFC deadlock detection. The default detection timer is 1.5 seconds. The value of detection time = granularity x detect-interval.
set class-of-service pfc-watchdog granularity <10 | 100>
set class-of-service pfc-watchdog code-point <cos> detect-interval <detect-interval> (On Trident3 platforms)
set class-of-service interface <interface-name> pfc-watchdog code-point <cos> detect-interval <detect-interval> (On Tomahawk3 platforms)
Step 4 Configure the restore time and restore action when PFC deadlock occurs. The default restore action is forward. The restore time = granularity x restore-interval.
set class-of-service pfc-watchdog restore-action <forward | drop>
set class-of-service pfc-watchdog code-point <cos> restore-interval <restore-interval> (On Trident3 platforms)
set class-of-service interface <interface-name> pfc-watchdog code-point <cos> restore-interval <restore-interval> (On Tomahawk3 platforms)
Step 5 Commit the configuration.
commit
Verifying the Configuration
After the configuration, use command run show pfc-watchdog config to view the configuration information about PFC watchdog.
admin@PICOS# run show pfc-watchdog config PORT ACTION QUEUE DETECTION TIME RESTORATION TIME ---------- ----------- ------------ ---------------- ------------------ te-1/1/25 drop 5 150 150 6 150 150 7 120 110 |
Use command run show pfc-watchdog stats to view the statistics information about PFC watchdog, including the number of PFC pause storms that have been detected and restored, as well as the number of packets that have been dropped, on the PFC queues on an interface.
admin@PICOS# run show pfc-watchdog stats
QUEUE STATUS STORM DETECTED/RESTORED TX OK/DROP TX LAST OK/DROP
------------ ----------- ------------------------- ---------------- -----------------
te-1/1/25:5 stormed 9/8 82072626556/0 32053822365/0
te-1/1/25:6 stormed 9/8 31504345475/0 32053822365/0
te-1/1/25:7 operational 0/0 0/0 0/0
|
In the show result,
STATUS: The status of PFC watchdog. The value could be operational or stormed.
operational: Currently under detection, no deadlock found.
stormed: Currently in a deadlock state.
STORM DETECTED: Queue deadlock counter.
STORM RESTORED: Queue Restore counter.
TX DROP and TX LAST DROP: Number of Tx packets dropped due to PFC deadlock.
TX OK and TX LAST OK: Number of Tx packets transmitted during deadlock (Forward action).