Configuring Priority Flow Control

About Priority Flow Control

Priority Flow Control (PFC) is used in lossless Ethernet to control the flow of data from a link partner for specific traffic priorities or classes specified as 'no-drop'. PFC Pause frames are transmitted to the link partner when certain queue thresholds are reached for a specific class or priority. PFC Pause frames are only local to the specific link but when traffic is suspended the congestion can cause PFC Pause frames to be generated on other links spreading the congestion. This can cause traffic for the priority or class to be suspended throughout the entire network for a time.

About Priority Flow Control Watchdog

Priority Flow Control Watchdog (PFCWD) 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.


Note

The PFC watchdog feature is supported only for no-drop queues.

Workflow of Priority Flow Control Watchdog

  • Monitors the PFC-enabled queues (no-drop queues) to identify the reception of an excessive number of PFC pause frames in a given interval (Watchdog interval).

  • Monitors when an excessive number of PFC frames are received and traffic on the corresponding queues is halted for a specified time interval (auto + fixed multiplier).

  • Initiates the shutdown timer and changes the queue's state to wait-to-shutdown.

  • Drops all data packets when the queue transitions to a shut state if interface multiplier time exceeds (if the interface multiplier is configured) or shutdown multiplier timer expire exceeds (if the interface multiplier is not configured).

  • Checks the queue for PFC frames and whether the traffic in the queue is still stuck at regular intervals (poll timer of 100ms) during the shutdown interval.

    • If traffic is stuck in the queue as PFC packets continue to arrive, the queue stays in the drop or shutdown state.

    • If the traffic is not stuck because the queue didn't receive any PFC frames, the queue reverts to the monitored state.

  • Checks if the queue is stuck because of PFC frames when the traffic is no longer stuck at regular intervals, the auto-restore timer starts.

    • If the queue receives PFC frames during the last auto-restore interval (poll timer * auto-restore multiplier), the auto-restore timer (secs) is reset at its expiration.

    • If the queue receives no PFC frames during the last auto-restore interval, the watchdog module restores the queue, and traffic resumes.

Prerequisites for Priority Flow Control

PFC has the following prerequisites:

  • You must be familiar with using modular QoS CLI.

  • You are logged on to the device.

Guidelines and Limitations for Priority Flow Control


Note

For scale information, see the release-specific Cisco Nexus 9000 Series NX-OS Verified Scalability Guide.

PFC has the following configuration guidelines and limitations:

  • If a QoS ACL is configured with DSCP match "X" for a lossless queue, all packets (IP, TCP, UDP, etc.) with DSCP "X" are mapped to the lossless queue.

  • The following guidelines apply to Cisco Nexus 9300-GX platform switches:

    Buffer allocation is based on the configuration irrespective of the operational state of the port.

    Buffers are allocated for no-drop operation when PFC operation mode turns on. No-drop buffers continue to remain allocated even if the interface goes down and the PFC operation mode remains on.

  • Adding the "pause buffer size threshold" configuration is optional for cable lengths that are less than 100 meters and it does need not to be configured.

  • Input queuing policy maps cannot have pause buffer and priority/bandwidth together.

  • For cable lengths greater than 100m, the "pause buffer size threshold" configuration is mandatory and it is required as part of the QoS policy configuration.

  • If PFC is enabled on a port or a port channel, it does not cause a port flap.

  • PFC configuration enables PFC in both the send (Tx) and receive (Rx) direction.

  • Configuration time quanta of the pause frames is not supported.

  • The configuration does not support pausing selected streams that are mapped to a particular traffic-class queue. All flows that are mapped to the class are treated as no-drop. It blocks out scheduling for the entire queue, which pauses traffic for all the streams in the queue. To achieve lossless service for a no-drop class, we recommend that you have only the no-drop class traffic on the queue.

  • When a no-drop class is classified based on 802.1p CoS x and assigned an internal priority value (qos-group) of y, we recommend that you use the internal priority value x to classify traffic on 802.1p CoS only, and not on any other field. The packet priority that is assigned is x if the classification is not based on CoS, which results in packets of internal priority x and y to map to the same priority x.

  • The PFC feature supports up to three no-drop classes of any maximum transmission unit (MTU) size. However, there is a limit on the number of PFC-enabled interfaces, based on the following factors:

    • MTU size of the no-drop class

    • Number of 10G and 40G ports

  • You can define the upper limit of any MTU in the system using the system jumbomtu command. The MTU range is from 1500 to 9216 bytes, and the default is 9216 bytes.

  • The interface QoS policy takes precedence over the system policy. PFC priority derivation also happens in the same order.

  • Ensure that you apply the same interface-level QoS policy on all PFC-enabled interfaces for both ingress and egress.


    Caution

    Irrespective of the PFC configuration, we recommend that you stop traffic before applying or removing a queuing policy that has strict-priority levels at the interface level or the system level.

  • To achieve end-to-end lossless service over the network, we recommend that you enable PFC on each interface through which the no-drop class traffic flows (Tx/Rx).

  • We recommend that you change the PFC configuration when there is no traffic. Otherwise, packets already in the Memory Management Unit (MMU) of the system may not get the expected treatment.

  • We recommend that you use default buffer sizes for no-drop classes or configure different input queuing policies suitable to 10G and 40G interfaces and the no-drop class MTU size. If the buffer size is specified through the CLI, it allocates the same buffer size for all ports irrespective of the link speed and MTU size. Applying the same pause buffer-size on 10G and 40G interfaces is not supported.

  • Do not enable WRED on a no-drop class because it results in drops in the egress queue.

  • Dynamic load balancing cannot be enabled for internal links with PFC. Disable DLB and enable RTAG7 load-balancing for internal links with the port-channel load-balance internal rtag7 command.

  • The dynamic load balancing (DLB) based hashing scheme is enabled by default on all internal links of a linecard. When DLB is enabled, no-drop traffic may experience an out-of-order packet delivery when congestion on internal links occurs and PFC is applied. If applications on the system are sensitive to out-of-order delivery, you can adjust for this event by disabling DLB at the qos-group level. Disable DLB by using the set dlb-disable action in the QoS policy-maps and the set qos-group action for no-drop classes.

    In the following example, assume that qos-group 1 is a no-drop class. DLB is disabled for this no-drop class by adding the set dlb-disable action and the set qos-group action. 

    switch(config)# policy-map p1
switch(config-pmap-qos)# class c1
switch(config-pmap-c-qos)# set qos-group 1
switch(config-pmap-c-qos)# set dlb-disable
switch(config-pmap-c-qos)# end
switch# show policy-map p1


  Type qos policy-maps
  ====================

  policy-map type qos p1
    class  c1
      set qos-group 1
      set dlb-disable

    Note

    The following Cisco Nexus platform switches do not support the set-dlb-disable command:

    • Cisco Nexus 9200 platform switches

    • Cisco Nexus 9300-EX/FX/FX2 platform switches

    • Cisco Nexus 9500 platform switches with -EX and -FX line cards

  • For VLAN-tagged packets, priority is assigned based on the 802.1p field in the VLAN tag and takes precedence over the assigned internal priority (qos-group). DSCP or IP access-list classification cannot be performed on VLAN-tagged frames.

  • For non VLAN-tagged frames, priority is assigned based on the set qos-group action provided by the ingress QoS policy. Classification is based on a QoS policy-allowed match condition such as precedence, DSCP, or access-list. Ensure that the pfc-cos value that is provided in the network-qos policy for this class is the same as the qos-group value in this case.

  • PFC is not supported for the Cisco Nexus 9408PC-CFP2 line card on Cisco Nexus 9500 platform switches.

  • Link level flow control and PFC are supported on Cisco Nexus 9300 Series switches and line cards that contain the ALE (Application Leaf Engine).

  • PFC on mode is used to support the hosts that support PFC but do not support the Data Center Bridging Capability Exchange Protocol (DCBXP).

  • DCBXP is supported on the following platforms:

    • Cisco Nexus 9200, 9300-EX, and 9300-FX2 platform switches

    • Cisco Nexus 9332C, 9332PQ, 9364C, 9372PX, 9372PX-E, and 9396PX switches

  • Only an exact match of the no-drop CoS is considered as a successful negotiation of PFC by the DCBXP.

  • The no lldp tlv-select dcbxp command is enhanced so that PFC is disabled for interfaces on both sides of back-to-back switches.

  • BUM traffic is not supported in no-drop PFC queues. Avoid marking multicast traffic as no-drop and sending it to these queues.

Default Settings for Priority Flow Control

Table 1. Default PFC Setting

Parameter

Default

PFC

Auto

Configuring Priority Flow Control

You can configure PFC on a per-port basis to enable the no-drop behavior for the CoS as defined by the active network QoS policy. PFC can be configured in one of these modes:

  • auto—Enables the no-drop CoS values to be advertised by the DCBXP and negotiated with the peer. A successful negotiation enables PFC on the no-drop CoS. Any failures because of a mismatch in the capability of peers causes the PFC not to be enabled. (Cisco NX-OS Release 7.0(3)I3(1) and later)

  • on—Enables PFC on the local port regardless of the capability of the peers.

  • off—Disables PFC on the local port.


Note

You can use the priority-flow-control override-interface mode off command to globally disable PFC on all interfaces regardless of the current interface configuration. This command, which is meant to be used during troubleshooting, allows you to quickly disable PFC without having to disable PFC on each interface. It is supported beginning with Cisco NX-OS Release 7.0(3)I4(2) and only for Cisco Nexus 9200 platform switches, Cisco Nexus 93108TC-EX and 93180YC-EX switches, and Cisco Nexus 9508 switches with the Cisco Nexus 9732C-EX line card.

Beginning with Cisco NX-OS Release 7.0(3)I4(5), this feature is supported on Cisco Nexus 9508 switches with Cisco Nexus 9636PQ line cards and Cisco Nexus 3164Q switches.

SUMMARY STEPS

  1. configure terminal
  2. interface type slot/port
  3. priority-flow-control mode [auto | off |on]
  4. show interface priority-flow-control

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

interface type slot/port

Example:

switch(config)# interface ethernet 2/5
switch(config-if)#

Enters interface mode on the interface specified.

Step 3

priority-flow-control mode [auto | off |on]

Example:

switch(config-if)# priority-flow-control mode on
switch(config-if)#

Sets PFC to the on mode.

Step 4

show interface priority-flow-control

Example:

switch# show interface priority-flow-control

(Optional) Displays the status of PFC on all interfaces.

Enabling Priority Flow Control on a Traffic Class

You can enable PFC on a particular traffic class.

SUMMARY STEPS

  1. configure terminal
  2. class-map type qos match { all | any } class-name
  3. match cos cos-value
  4. match dscp dscp-value
  5. exit
  6. policy-map type qos policy-name
  7. class class-name
  8. set qos-group qos-group-value
  9. exit
  10. exit
  11. policy-map type network-qos policy-name
  12. class type network-qos class-name
  13. pause pfc-cos value [ receive ]
  14. exit
  15. exit
  16. system qos
  17. service-policy type network-qos policy-name
  18. exit
  19. interface ethernet slot / number
  20. priority-flow-control mode { auto | on | off }
  21. service-policy type qos input policy-name
  22. exit

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

class-map type qos match { all | any } class-name

Example:

switch(config)# class-map type qos c1
switch(config-cmap-qos)#

Creates a named object that represents a class of traffic. Class-map names can contain alphabetic, hyphen, or underscore characters, are case sensitive, and can be up to 40 characters.

match { all | any } : Default is match all (if multiple matching statements are present all of them must be matched).

Step 3

match cos cos-value

Example:

switch(config-cmap-qos)# match cos 2
switch(config-cmap-qos)#

Specifies the CoS value to match for classifying packets into this class. You can configure a CoS value in the range of 0 to 7.

Step 4

match dscp dscp-value

Example:

switch(config-cmap-qos)# match dscp 3
switch(config-cmap-qos)#

Specifies the DSCP value to match for classifying packets into this class. You can configure a DSCP value in the range of 0 to 63 or the listed values.

Step 5

exit

Example:

switch(config-cmap-qos)# exit
switch(config)#

Exits class-map mode and enters global configuration mode.

Step 6

policy-map type qos policy-name

Example:

switch(config)# policy-map type qos p1
switch(config-pmap-qos)#

Creates a named object that represents a set of policies that are to be applied to a set of traffic classes. Policy-map names can contain alphabetic, hyphen, or underscore characters, are case sensitive, and can be up to 40 characters.

Step 7

class class-name

Example:

switch(config-pmap-qos)# class c1
switch(config-pmap-c-qos)#

Associates a class map with the policy map and enters the configuration mode for the specified system class.

Note

 

The associated class map must be the same type as the policy map type.

Step 8

set qos-group qos-group-value

Example:

switch(config-pmap-c-qos)# set qos-group 3
switch(config-pmap-c-qos)#

Configures one or more qos-group values to match on for classification of traffic into this class map. There is no default value.

Step 9

exit

Example:

switch(config-pmap-c-qos)# exit
switch(config-pmap-qos)#

Exits the system class configuration mode and enters policy-map mode.

Step 10

exit

Example:

switch(config-pmap-qos)# exit
switch(config)#

Exits policy-map mode and enters global configuration mode.

Step 11

policy-map type network-qos policy-name

Example:

switch(config)# policy-map type network-qos pfc-qos
switch(config-pmap-nqos)#

Creates a named object that represents a set of policies that are to be applied to a set of traffic classes. Policy-map names can contain alphabetic, hyphen, or underscore characters, are case sensitive, and can be up to 40 characters.

Step 12

class type network-qos class-name

Example:

switch(config-pmap-nqos)# class type network-qos nw-qos3
switch(config-pmap-nqos-c)#

Associates a class map with the policy map, and enters the configuration mode for the specified system class.

Note

 

The associated class map must be the same type as the policy map type.

Step 13

pause pfc-cos value [ receive ]

Example:

switch(config-pmap-nqos-c)# pause pfc-cos 3 receive
switch(config-pmap-nqos-c)#

PFC sends a pause frame that indicates which CoS value needs to be paused. Only PFC receive is enabled for the list of PCF CoS values.

receive : When this optional keyword is used, PFC only receives and honors pause frames. PFC will never send pause frames. This is known as "Asymmetric PFC".

Note

 

Although not required, the pause pfc-cos value should match the qos-group-value in the set qos-group command. See the set qos-group command in steps 8 above.

Step 14

exit

Example:

switch(config-pmap-nqos-c)# exit
switch(config-pmap-nqos)#

Exits configuration mode and enters policy-map mode.

Step 15

exit

Example:

switch(config-pmap-nqos)# exit
switch(config)#

Exits policy-map mode and enters global configuration mode.

Step 16

system qos

Example:

switch(config)# system qos
switch(config-sys-qos)#

Enters system class configuration mode.

Step 17

service-policy type network-qos policy-name

Example:

switch(config-sys-qos)# service-policy type network-qos pfc-qos

Applies the policy map of type network-qos at the system level or to the specific interface.

Step 18

exit

Example:

switch(config-sys-qos)# exit
switch(config)#

Exits policy-map mode and enters global configuration mode.

Step 19

interface ethernet slot / number

Example:

switch(config)# interface ethernet 1/1
switch(config-if)#

Enters the ethernet interface configuration mode for the selected slot and chassis number.

Step 20

priority-flow-control mode { auto | on | off }

Example:

switch(config-if)# priority-flow-control mode on
switch(config-if)#

Enables the priority flow control policy for the interface.

Step 21

service-policy type qos input policy-name

Example:


switch(config-if)# service-policy type qos input p1

Adds classification to the interface ensuring that packets matching the previously configured CoS or DSCP values are classified in the correct QoS group.

Step 22

exit

Example:

switch(config-if)# exit
switch(config)#

Exits the ethernet interface mode and enters the global configuration mode.

Configuring a Link Level Flow Control Watchdog and Priority Flow Control Watchdog

Link Level Flow Control Watchdog (LLFCWD) is enabled globally by default. LLFCWD on an interface is enabled automatically when PFC and PFCWD are configured on the interface. If an LLFC packet is seen on a PFC/PFCWD configured interface that doesn’t have LLFC configured, the LLFC watchdog is triggered.

PFCWD interval and PFCWD multiplier CLI commands are used to configure the LLFCWD interval and multipliers. Use this procedure to configure the LLFC watchdog interval and the multiplier used to restore the no-drop queue.


Note

PFC watchdog is not supported on Cisco Nexus 9500 platform switches with Cisco Nexus 9400, 9500 and 9600 line cards, with the exception of Cisco Nexus 9636PQ line cards (that support the PFC watchdog feature).


Note

Ingress drops provide statistics of PFC watchdog dropped packets on the front panel ports.

Note

For Cisco Nexus 9200 platform switches, Cisco Nexus 9300-EX/FX/FX2 platform switches, and Cisco Nexus 9500 platform switches with -EX or -FX line cards, one of the following calculations is performed to determine when the queue is moved to the shutdown state:

If the interface multiplier is configured, the following calculation is used:

priority-flow-control watch-dog interval value * priority-flow-control watch-dog internal-interface-multiplier multiplier

If the interface multiplier is not configured, the watchdog shutdown multiplier is used instead:

priority-flow-control watch-dog interval value * priority-flow-control watch-dog shutdown-multiplier multiplier

Before you begin

Consider the following before configuring the Link Level Flow Control Watchdog Interval:

  • Link Level Flow Control Watchdog is supported on the following Cisco Nexus 9000 Series platform switches and line cards:

    • N9K-C9232C

    • N9K-C9236C

    • N9K-C92304QC

    • N9K-X9736C-EX

    • N9K-X9732C-EX

    • N9K-X9732C-EXM

    • N9K-X97160YC-EX

    • N9K-C93180YC-FX3S

    • N9K-C93108TC-FX3P

  • PFC must be enabled at the interface. PFCWD must be enabled at the interface and globally. LLFC shouldn’t be configured on the same interface. For more information on PFC, see Configuring Priority Flow Control.


    Note

    PFC watchdog uses a command to send a syslog message indicating that the queue is "stuck" (priority-flow-control watch-dog-interval on disable-action ). If this command is invoked on a PFC interface, the queue isn’t shut down but instead, the syslog message is generated. When the LLFC watchdog feature is enabled, and if a link level flow control packet is received on an interface, the queue is shut even with the disable-action command for PFC watchdog is enabled.

  • Auto-restore and fixed restore should never be configured to 0.

  • If LLFC is enabled on the interface, then LLFC WD is disabled.

SUMMARY STEPS

  1. configure terminal
  2. priority-flow-control auto-restore multiplier value
  3. priority-flow-control fixed-restore multiplier value
  4. priority-flow-control watch-dog-interval {on | off}
  5. priority-flow-control watch-dog interval value
  6. priority-flow-control watch-dog shutdown-multiplier multiplier
  7. (Optional) priority-flow-control watch-dog internal-interface-multiplier multiplier
  8. (Optional) show queuing pfc-queue [interface interface-list] [module module] [detail]
  9. (Optional) show queuing llfc-queue [interface interface-list] [module module] [detail]
  10. (Optional) clear queuing pfc-queue [interface] [ethernet|ii] [intf-name]
  11. (Optional) clear queuing llfc-queue [interface interface-list] [module module]
  12. (Optional) priority-flow-control recover interface [ethernet|ii] [intf-name] [qos-group <0-7>]

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

priority-flow-control auto-restore multiplier value

Configures a value for the auto-restore multiplier, which is calculated by multiplying the set PFC WD interval time. The range is from 0 to 100.

Note

 

The auto-restore multiplier should never be configured to 0.

When the LLFC watchdog no-drop queue is restored, a system logging message entry is created to record the conditions of the queue. The following is an example of the message: 

Error Message TAHUSD-SLOT#-2- TAHUSD_SYSLOG_LLFCWD_QUEUE_RESTORED : [chars]
Description : NO DROP Queue Restored due to LLFC WatchDog timer expiring message

This command is applicable for both LLFCWD and PFCWD.

Step 3

priority-flow-control fixed-restore multiplier value

Configures a value for the PFC fixed-restore multiplier.

Step 4

priority-flow-control watch-dog-interval {on | off}

Example:

switch(config)# priority-flow-control watch-dog-interval on

Globally enables or disables the PFC watchdog interval for all interfaces. This command should be configured at global and also at an interface.

See the following example of the command configured at global:

switch(config)# priority-flow-control watch-dog-interval on

See the following example of the command configured at an interface:

switch(config)# interface ethernet 7/5
switch(config-if)# priority-flow-control watch-dog-interval on

Note

 

You can use this same command in interface configuration mode to enable or disable the PFC watchdog interval for a specific interface.

This command is applicable for both LLFCWD and PFCWD.

See the following example of the command configured at an interface with a specific shutdown multiplier value (Cisco NX-OS Release 7.0(3)I7(4) and later releases): 

switch(config)# int e1/36
switch(config-if)# priority-flow-control watch-dog-interval on interface-multiplier 10

Note

 

Range of values for interface-multiplier is 1 - 10.

Step 5

priority-flow-control watch-dog interval value

Example:

switch(config)# priority-flow-control watch-dog interval 200

Specifies the watchdog interval value of all queues and ports for which this configuration is enabled. The range is from 100 to 1000 milliseconds.

Note

 

This command is applicable for both LLFCWD and PFCWD.

Step 6

priority-flow-control watch-dog shutdown-multiplier multiplier

Example:

switch(config)# priority-flow-control watch-dog shutdown-multiplier 5

Specifies when to declare the PFC queue as stuck shutdown multiplier * poll interval. The range is from 1 to 10, and the default value is 1.

Note

 

When the PFC queue is declared as stuck, a syslog entry is created to record the conditions of the PFC queue. (Cisco NX-OS Release 7.0(3)I7(4) and later releases)

Step 7

(Optional) priority-flow-control watch-dog internal-interface-multiplier multiplier

Example:

switch(config)# priority-flow-control watch-dog internal-interface-multiplier 5
 (Optional)

Configures a PFC watchdog poll-interval multiplier for HiGig™ interfaces. The range is from 0 to 10, and the default value is 2. A value of 0 disables this feature on HiGig™ interfaces.

Note

 

This command is only applicable for EoR switches.

Step 8

(Optional) show queuing pfc-queue [interface interface-list] [module module] [detail]

Example:

switch(config)# sh queuing pfc-queue interface ethernet 1/1 detail
 (Optional)

Displays the PFCWD statistics.

Step 9

(Optional) show queuing llfc-queue [interface interface-list] [module module] [detail]

Example:

switch(config)# show queuing llfc-queue interface ethernet 1/1 detail
 (Optional)

Displays the LLFCWD statistics. See the output example at the end of this procedure.

Step 10

(Optional) clear queuing pfc-queue [interface] [ethernet|ii] [intf-name]

Example:

switch(config)# clear queuing pfc-queue interface ethernet 1/1
 (Optional)

Clears the environment variable PFCWD statistics.

Step 11

(Optional) clear queuing llfc-queue [interface interface-list] [module module]

Example:

switch(config)# clear queuing llfc-queue interface ethernet 1/1
 (Optional)

Clears the LLFCWD queue statistics.

Step 12

(Optional) priority-flow-control recover interface [ethernet|ii] [intf-name] [qos-group <0-7>]

Example:

switch# priority-flow-control recover interface ethernet 1/1 qos-group 3
 (Optional)

Recovers the interface manually.

Example

Beginning with Cisco NX-OS Release 7.0(3)I6(1), on Cisco Nexus 9200, 9300, 9300-EX, and 9500 platform switches, using the detail option, you can account for Ingress drops. 

| QOS GROUP 1 [Active] PFC [YES] PFC-COS [1]                               
+----------------------------------------------------+
|                               |  Stats             |
+----------------------------------------------------+
|                       Shutdown|                   0|
|                       Restored|                   0|
|             Total pkts drained|                   0|
|             Total pkts dropped|                   0|
|   Total pkts drained + dropped|                   0|
|         Aggregate pkts dropped|                   0|
|     Total Ingress pkts dropped|                   0| ===>>>>>Ingress
| Aggregate Ingress pkts dropped|                   0|===>>>>Ingress
+——————————————————————————+

The following example shows detail output of the show queuing llfc-queue command for an Ethernet 1/1 interface: 

switch# show queuing llfc-queue interface 1/1 detail

slot  1
=======
+----------------------------------------------------+
Global watch-dog interval [Enabled]
+----------------------------------------------------+
+----------------------------------------------------+
Global LLFC watchdog configuration details

LLFC watchdog poll interval                 : 100 ms
LLFC watchdog auto-restore multiplier       : 10
LLFC watchdog fixed-restore multiplier      : 0
+----------------------------------------------------+

+----------------------------------------------------+
Ethernet1/1 Interface LLFC watchdog: [Enabled]
+----------------------------------------------------+
+----------------------------------------------------+
| QOS GROUP 6 [Active] LLFC [YES] LLFC-COS [6]
+----------------------------------------------------+
|                               |  Stats             |
+----------------------------------------------------+
|                       Shutdown|                   1|
|                       Restored|                   1|
|             Total pkts drained|                 554|
|             Total pkts dropped|            56093783|
|   Total pkts drained + dropped|            56094337|
|         Aggregate pkts dropped|            56094337|
|     Total Ingress pkts dropped|                   0|
| Aggregate Ingress pkts dropped|                   0|
+----------------------------------------------------+

Configuring Pause Buffer Thresholds and Queue Limit Using Ingress Queuing Policy

The pause buffer thresholds specified in the network-qos policy are shared by all the ports in the system. However, there are situations where a few ports may need different thresholds (such as long distance connections). An ingress queuing policy can be used for this purpose.

An ingress queuing policy also allows the configuration of the queue-limit to restrict the amount of shared buffer that can be used in addition to the reserved pause buffer by the no-drop class.

Each no-drop class is mapped internally to one of the port's priority-group in the ingress direction. The configured pause buffer thresholds and queue-limit are applied to the priority-group associated with the class.


Note

Adding pause buffer size threshold configuration is optional for cable lengths that are less than 100 meters and it need not be configured.

For cable lengths that are greater than 100m, the pause buffer size threshold configuration is mandatory and it is required as part of the QoS policy configuration.


Note

About queue limits for 100G enabled devices (such as the Cisco Nexus 9300 platform switch with the N9K-M4PC-CFP2 GEM):

  • The maximum dynamic queue-limit alpha value supported by the device might be greater that 8. However 8 is the maximum alpha value supported. Configuring the alpha value to a value greater than 8 is overridden by the maximum alpha value of 8.

    No message is issued when the alpha value is overridden.

  • The static queue-limit has a maximum of 20,000 cells. Any value specified greater than the maximum 20,000 cell limit is overridden by the 20,000 cell limit.

    No message is issued when the cell limit is overridden.

SUMMARY STEPS

  1. configure terminal
  2. policy-map type queuing policy-map-name
  3. class type queuing c-in-q1
  4. pause buffer-size buffer-size pause threshold xoff-size resume threshold xon-size
  5. no pause buffer-size buffer-size pause threshold xoff-size resume threshold xon-size
  6. queue-limit queue size [dynamic dynamic threshold]

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Enters global configuration mode.

Step 2

policy-map type queuing policy-map-name

Enters policy-map queuing class mode and identifies the policy map assigned to the type queuing policy map.

Step 3

class type queuing c-in-q1

Attaches the class map of type queuing and then enters policy-map class queuing mode. Class queuing names are listed in the System-Defined Type queuing Class Maps table.

Note

 

The qos-group associated with the class must be defined as a no-drop class in the network-qos policy applied in the system qos.

Note

 

Up to eight ingress queues are supported for the Cisco Nexus 9636C-R and 9636Q-R line cards and the Cisco Nexus 9508-FM-R fabric module (in a Cisco Nexus 9508 switch). The range is from c-in-8q-q-default to c-in-8q-q1 through 7.

Step 4

pause buffer-size buffer-size pause threshold xoff-size resume threshold xon-size

Specifies the buffer threshold settings for pause and resume.

Step 5

no pause buffer-size buffer-size pause threshold xoff-size resume threshold xon-size

Removes the buffer threshold settings for pause and resume.

Step 6

queue-limit queue size [dynamic dynamic threshold]

(Optional) Specifies either the static or dynamic shared limit available to the ingress priority-group. The static queue limit defines the fixed size to which the priority-group can grow. The dynamic queue limit allows the priority-group's threshold size to be decided depending on the number of free cells available, in terms of the alpha value.

Note

 

Cisco Nexus 9200 platform switches only support a class level dynamic threshold configuration with respect to the alpha value. This means that all ports in a class share the same alpha value.

Note

 

The queue limit for the Cisco Nexus 9636C-R and 9636Q-R line cards and the Cisco Nexus 9508-FM-R fabric module (in a Cisco Nexus 9508 switch) can be entered as a percent or in bytes/kbytes/mbytes/gbytes. For example, queue-limit percent 1 or queue-limit bytes 100.

Verifying the Priority Flow Control Configuration

To display the PFC configuration, perform the following task:

Command

Purpose

show interface priority-flow-control [module number]

Displays the status of PFC on all interfaces or on specific modules.

Configuration Examples for Priority Flow Control

The following example shows how to configure PFC: 

configure terminal
interface ethernet 5/5
priority-flow-control mode on

The following example shows how to enable PFC on a traffic class: 

switch(config)# class-map type qos c1
switch(config-cmap-qos)# match cos 3
switch(config-cmap-qos)# exit
switch(config)# policy-map type qos p1
switch(config-pmap-qos)# class type qos c1
switch(config-pmap-c-qos)# set qos-group 3
switch(config-pmap-c-qos)# exit
switch(config-pmap-qos)# exit
switch(config)# class-map type network-qos match-any c1
switch(config-cmap-nqos)# match qos-group 3
switch(config-cmap-nqos)# exit
switch(config)# policy-map type network-qos p1
switch(config-pmap-nqos)# class type network-qos c-nq1
switch(config-pmap-nqos-c)# pause pfc-cos 3
switch(config-pmap-nqos-c)# exit
switch(config-pmap-nqos)# exit
switch(config)# system qos
switch(config-sys-qos)# service-policy type network-qos p1
The following example shows how to configuring the PFC mode and its policies which is a prerequisite for PFC watchdog:
Watchdog is enabled by default, with system default values of:
 
Watchdog interval = 100 ms
 
Shutdown multiplier = 1
 
Auto-restore multiplier = 10
The following example shows how to check PFC watchdog statistics:
switch# show queuing pfc-queue interface ethernet 1/23
 
slot  1
=======
+----------------------------------------------------+
Global watch-dog interval [Enabled]
Forced Global watch-dog   [Enabled]
+----------------------------------------------------+
 
+----------------------------------------------------+
Global PFC watchdog configuration details
 
PFC watchdog poll interval                 : 100 ms
PFC watchdog shutdown multiplier           : 1
PFC watchdog auto-restore multiplier       : 10
PFC watchdog fixed-restore multiplier      : 0
PFC watchdog internal-interface multiplier : 2
+----------------------------------------------------+
 
+-------------------------------------------------------------+
| Port              PFC Watchdog  (VL bmap)  State (Shutdown) |
+-------------------------------------------------------------+
  Ethernet1/23      Enabled       ( 0x8 )   - - - - Y - - -  >>>>>>>>>>>>>>>> The Queue is marked as SHUT
  
 
switch# show queuing pfc-queue interface ethernet 1/23 detail
 
slot  1
=======
+----------------------------------------------------+
Global watch-dog interval [Enabled]
Forced Global watch-dog   [Enabled]
+----------------------------------------------------+
 
+----------------------------------------------------+
Global PFC watchdog configuration details
 
PFC watchdog poll interval                 : 100 ms
PFC watchdog shutdown multiplier           : 1
PFC watchdog auto-restore multiplier       : 10
PFC watchdog fixed-restore multiplier      : 0
PFC watchdog internal-interface multiplier : 2
+----------------------------------------------------+
 
+----------------------------------------------------+
Ethernet1/23 Interface PFC watchdog: [Enabled]
Disable-action                     : No
PFC watch-dog interface-multiplier : 0
+----------------------------------------------------+
+----------------------------------------------------+
| QOS GROUP 3 [Shutdown] PFC [YES] PFC-COS [3]
+----------------------------------------------------+
|                               |  Stats             |
+----------------------------------------------------+
|                       Shutdown|                   1|
|                       Restored|                   0|
|             Total pkts drained|                   0|
|             Total pkts dropped|                   0|
|   Total pkts drained + dropped|                   0|
|         Aggregate pkts dropped|                   0|
|     Total Ingress pkts dropped|                1924|    >>>>>>>>> Account for Ingress drops here
| Aggregate Ingress pkts dropped|                1924| 
+----------------------------------------------------+

Configuring a No-Drop Policy

The following example shows how to configure a no-drop policy and attach the policy to a session policy: 


Device# configure terminal
Device(config)# class-map type network-qos class1
Device(config-cmap-nq)# match qos-group 1
Device(config-cmap-nq)# policy-map type network-qos my_network_policy
Device(config-pmap-nq)# class type network-qos class1
Device(config-pmap-nq-c)# pause pfc-cos 2
Device(config-pmap-nq-c)# system qos
Device(config-sys-qos)# service-policy type network-qos my_network_policy
Device# show running ipqos

Classifying Traffic to a No-Drop Class

The following example shows how to create a QoS policy to map all the traffic to the no-drop class: 


Device# configure terminal
Device(config)# class-map type qos class1
Device(config-cmap-qos)# match cos 2
Device(config-cmap-qos)# policy-map type qos my_qos_policy
Device(config-pmap-qos)# class type qos class1
Device(config-pmap-c-qos)# set qos-group 1
Device(config-pmap-c-qos)# interface e1/5
Device(config-sys-qos)# service-policy type qos input my_qos_policy
Device(config-sys-qos)#

Add the queuing policy that guarantees the bandwidth for qos-group 1 and apply that under system-qos as outlined in the following example: 


policy-map type queuing my_queuing_policy
class type queuing c-out-q-default
bandwidth percent 1
class type queuing c-out-q3
bandwidth percent 0
class type queuing c-out-q2
bandwidth percent 0
class type queuing c-out-q1
bandwidth percent 99
  
system qos
  service-policy type queuing output my_queuing_policy

In the above example, c-out-q1 by default matches the traffic on qos-group 1. Therefore, the non-default class-map for queuing which matches qos-group 1 is not needed. For further information on configuring queuing, see Configuring Queuing.

For LLFC to be enabled, you need to configure the no-drop policy on network-qos. The buffering module needs to inform the MAC module to generate pause (either LLFC or PFC based on the interface level configuration). PFC negotiation to the adapter is by using DCBX. LLFC or PFC is controlled by the configuration on the interfaces. For example, the flow-control send and receive on enables LLFC on the interfaces and the priority-flow-control mode on enables PFC on the interfaces.

If DCBX is supported, auto mode negotiates the PFC with the adapter. This is the interface level configuration to enable LLFC or PFC but regardless of it, you have to configure network-qos level pause configuration for LLFC to work. Even if the traffic is classified to qos-group 1 but when it generates pause, it generates LLFC based on the interface level configuration.