PFC Configuration Example
- Tracy Yang (Unlicensed)
Priority Flow Control or PFC is a kind of flow control mechanism. The advantage of PFC over traditional Flow Control mechanisms is that PFC provides flow control based on per code point (priority). In other words, PFC provides a more granular form of flow control. This means that if traffic from one particular priority suffers from congestion, only that traffic is paused until congestion clears away, whereas traffic for other priorities continues unhindered. On each physical port, there are 8 (0 to 7) Class of Service (CoS) queues, if congestion is detected on the egress physical port, the ingress port will send a PAUSE frame to the transmitting node to pause transmission until the receiving node is ready to accept packets again. PFC applies only to packets entering a port.
PFC has a higher priority than flow control. So, for example, if both flow control and PFC are configured on the same port, PFC will have precedence over traditional flow control.
PFC uses the IEEE 802.1p CoS values in the IEEE 802.1Q VLAN tag to generate the flow control frame with corresponding priority on ingress physical port when egress physical port suffers congestion. It indicates the ingress port needs COS classifier configuration.
PFC configuration is applied using PFC profiles. The following is an example of PFC configuration.
Configure PFC Profile
PFC is disabled when drop value is set to true and enabled when drop value is set to false. The default value of drop is false.
For example as below, PFC is enabled on 0，1，3，5，6，7 code-point by default, PFC is disabled on 2，4 code-point.
admin@XorPlus# set class-of-service pfc-profile pfc1 code-point 2 drop true admin@XorPlus# set class-of-service pfc-profile pfc1 code-point 4 drop true admin@XorPlus# commit Commit OK. Save done.
Apply PFC Profile to Port
admin@XorPlus# set class-of-service interface ge-1/1/1 pfc-profile pfc1 admin@XorPlus# commit Commit OK. Save done.
Show PFC Frame Statistics on Port
The class 0~7 in PFC frame corresponds to the following "802.1P" item. The value of ”RxPFC“ item will be incremented by 1 if ge-1/1/1 receives a PFC frame. The value of ”TxPFC“ item will be incremented by 1 if ge-1/1/1 sends out a PFC frame.
admin@XorPlus# run show class-of-service interface ge-1/1/1 Interface : ge-1/1/1 802.1P Priority Flow Control RxPFC TxPFC ----------- --------------------- --------------- --------------- 0 true 0 0 1 true 0 0 2 false 0 0 3 true 0 0 4 false 0 0 5 true 0 0 6 true 0 0 7 true 0 0 trust mode : no-trust Default ieee-802.1 : 0 Default dscp : 0 Default inet-precedence : 0 Local-priority Queue-Schedule Code-points -------------- --------------------------- ------------------------- 0 SP,0kbps 1 SP,0kbps 2 SP,0kbps 3 SP,0kbps 4 SP,0kbps 5 SP,0kbps 6 SP,0kbps 7 SP,0kbps
Apply classifier based on IEEE 802.1P to ingress port
admin@XorPlus# set class-of-service classifier c1 admin@XorPlus# set class-of-service interface ge-1/1/1 classifier "c1" admin@XorPlus# commit Commit OK. Save done. admin@XorPlus# run show class-of-service interface ge-1/1/1 Interface : ge-1/1/1 802.1P Priority Flow Control RxPFC TxPFC ----------- --------------------- --------------- --------------- 0 true 0 0 1 true 0 0 2 false 0 0 3 true 0 0 4 false 0 0 5 true 0 0 6 true 0 0 7 true 0 0 trust mode : no-trust Default ieee-802.1 : 0 Default dscp : 0 Default inet-precedence : 0 Local-priority Queue-Schedule Code-points -------------- --------------------------- ------------------------- 0 SP,0kbps 0 1 SP,0kbps 1 2 SP,0kbps 2 3 SP,0kbps 3 4 SP,0kbps 4 5 SP,0kbps 5 6 SP,0kbps 6 7 SP,0kbps 7
notice
Since version of 2.9.0, PFC has port limit. Different platform can support different port number with PFC .The configuration field as shown below :
| Chip | Trident | Trident2 | Trident2 plus | Helix4 | Tomahawk(perxpe) | |
| 1G | max PFC ports | 18 | ||||
| 10G/25G | max PFC ports | 32 | 42 | 56 | 11 | 12 |
| 40G | max PFC ports | 19 | 25 | 33 | 7 | |
| 100G | max PFC ports | 3 |
In addition,if a 40G port configures PFC, the valid PFC port number of 10G/25G will reduce 2 .
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