Priority Flow Control Overview
|
Feature Name |
Release Information |
Feature Description |
|---|---|---|
|
Priority Flow Control on Cisco 8808 and Cisco 8812 Modular Chassis Line Cards |
Release 7.5.3 |
Priority Flow Control is now supported on the following line card in the buffer-internal mode:
The feature is supported in the buffer-internal and buffer-extended modes on:
Apart from the buffer-external mode, support for this feature now extends to the buffer-internal mode on the following line cards:
|
|
Shortlink Priority Flow Control |
Release 7.3.3 |
This feature and the hw-module profile priority-flow-control command are supported on 88-LC0-36FH line card. |
|
Priority Flow Control Support on Cisco 8800 36x400 GbE QSFP56-DD Line Cards (88-LC0-36FH-M) |
Release 7.3.15 |
This feature and the hw-module profile priority-flow-control command are supported on 88-LC0-36FH-M and 8800-LC-48H line cards. All previous functionalities and benefits of this feature are available on these line cards. However, the buffer-internal mode is not supported.In addition, to use the buffer-extended mode on these line cards, you are required to configure the performance capacity or headroom values. This configuration requirement ensures that you can better provision and balance workloads to achieve lossless behavior, which in turn ensures efficient use of bandwidth and resources. |
|
Priority Flow Control |
Release 7.3.1 |
This feature and the hw-module profile priority-flow-control command are not supported. |
Priority-based Flow Control (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.x Flow Control (pause frames) or link-level flow control (LFC). However, PFC functions on a per class-of-service (CoS) basis.
During congestion, PFC sends a pause frame to indicate the CoS value to pause. A PFC pause frame contains a 2-octet timer value for each CoS that indicates the length of time to pause the traffic. The unit of time for the timer is specified in pause quanta. A quanta is the time required for transmitting 512 bits at the speed of the port. The range is from 0 through 65535 quanta.
PFC asks the peer to stop sending frames of a particular CoS value by sending a pause frame to a well-known multicast address. This pause frame is a one-hop frame and isn't forwarded when received by the peer. When the congestion mitigates, the router stops sending the PFC frames to the upstream node.
You can configure PFC for each line card using the hw-module profile priority-flow-control command in one of two modes:
-
buffer-internal
-
buffer-extended
 Note |
PFC threshold configurations are deprecated in pause command. Use the hw-module profile priority-flow-control command to configure PFC threshold configurations. |
 Tip |
You can programmatically retrieve the operational state of the PFC configuration using |
Related Topics
buffer-internal mode
Use this mode if PFC-enabled devices aren't more than 1 km apart.
You can set values for pause-threshold, headroom (both related to PFC), and ECN for the traffic class using the hw-module profile priority-flow-control command in this mode. The buffer-internal configuration applies to all ports that the line card hosts, which mean that you can configure a set of these values per line card.
The existing queue limit and ECN configuration in the queueing policy attached to the interface has no impact in this mode.
The effective queue limit for this mode = pause-threshold + headroom (in bytes)
Restrictions and Guidelines
The following restrictions and guidelines apply while configuring the PFC threshold values using the buffer-internal mode.
-
The PFC feature isn't supported on fixed chassis systems.
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Ensure that there’s no breakout configured on a chassis that has the PFC configured. Configuring PFC and breakout on the same chassis may lead to unexpected behavior, including traffic loss.
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The feature isn't supported on bundle and non-bundle sub-interface queues.
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The feature is supported on 40GbE, 100 GbE, and 400 GbE interfaces.
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The feature isn't supported in the 4xVOQ queueing mode.
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The feature isn't supported when sharing of VOQ counters is configured.
buffer-extended mode
Use this mode for PFC-enabled devices with long-haul connections.
You can set the value for pause-threshold using the hw-module profile priority-flow-control command in this mode. You must, however, configure the queuing policy attached to the interface to set the ECN and queueing limits. The buffer-extended configuration applies to all ports that the line card hosts, which mean that you can configure a set of these values per line card.
Configuration Guidelines
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Important points while configuring the buffer-extended mode on 88-LC0-36FH-M line cards:
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Apart from pause-threshold, you must also configure values for headroom.
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The headroom value range is from 4 through 75000.
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Specify pause-threshold and headroom values in units of kilobytes (KB) or megabytes (MB).
-
-
Important points while configuring the buffer-extended mode on 8800-LC-48H line cards:
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Configure values only for pause-threshold. Don’t configure headroom values.
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Configure pause-threshold in units of milliseconds (ms) or microseconds.
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Don’t use units of kilobytes (KB) or megabytes (MB) units, even though the CLI displays them as options. Only use units of milliseconds (ms) or microseconds.
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(Also see Configure Priority Flow Control)
Important Considerations
-
If you configure PFC values in the buffer-internal mode, then the ECN value for the line card is derived from the buffer-internal configuration. If you configure PFC values in the buffer-extended mode, then the ECN value is derived from the policy map. (For details on the ECN feature, see Explicit Congestion Notification.)
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The buffer-internal and buffer-extended modes can't coexist on the same line card.
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For Cisco 8808 and Cisco 8812 chassis, configure PFC on all line cards in the chassis, regardless of whether you're configuring the buffer-internal or buffer-extended mode. Otherwise, your network may experience traffic loss.
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If you add or remove traffic-class actions on a line card, you must reload the line card.
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When using the buffer-internal mode, you can change values of the following parameters without having to reload the line card. However, if you add a new traffic class and configure these values for the first time on that traffic class, you must reload the line card for the values to come into effect.
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pause-threshold
-
headroom
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ECN
-
-
If you add or remove ECN configuration using the hw-module profile priority-flow-control command, you must reload the line card for the ECN changes to take effect.
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The PFC threshold value ranges for the buffer-internal mode are as follows.
Threshold
Configured (bytes)
pause (min)
307200
pause (max)
422400
headroom (min)
345600
headroom (max)
537600
ecn (min)
153600
ecn (max)
403200
-
For a traffic-class, the ECN value must always be lesser than the configured pause-threshold value.
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The combined configured values for pause-threshold and headroom must not exceed 844800 bytes. Else, the configuration is rejected.
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The pause-threshold value range for buffer-extended mode is from 2 milliseconds (ms) through 25 ms and from 2000 microseconds through 25000 microseconds.
Hardware Support for Priority Flow Control
The table lists the PIDs that support PFC per release and the PFC mode in which the support is available.
|
Release |
PID |
PFC Mode |
|---|---|---|
|
Release 7.5.3 |
88-LC0-36FH |
buffer-extended and buffer-internal |
|
88-LC0-36FH-M |
buffer-extended and buffer-internal |
|
|
8800-LC-48H |
buffer-extended and buffer-internal |
|
|
88-LC0-34H14FH |
buffer-internal |
|
|
Release 7.3.15 |
|
buffer-extended |
|
Release 7.0.11 |
8800-LC-48H |
buffer-internal |
Configure Priority Flow Control
Note |
The system enables shortlink PFC by default when you enable PFC. |
YANG Data Model |
You can programmatically manage the flow control parameters using the |
Configuration Example
You must accomplish the following to complete the PFC configuration:
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Enable PFC at the interface level.
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Configure ingress classification policy.
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Attach the PFC policy to the interface.
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Configure PFC threshold values using either the buffer-internal or buffer-extended mode.
Router# configure
Router(config)# priority-flow-control mode on
/*Configure ingress classification policy*/
Router(config)# class-map match-any prec7
Router(config-cmap)# match precedence
Router(config)# class-map match-any tc7
/*Ingress policy attach*/
Router(config-if)# service-policy input QOS_marking
/*Egress policy attach*/
Router(config-if)# service-policy output qos_queuing
Router(config-pmap-c)# exit
Router(config-pmap)# exit
Router(config)#show controllers npu priority-flow-control location <loc>
Running Configuration
*Interface Level*
interface HundredGigE0/0/0/0
priority-flow-control mode on
*Ingress:*
class-map match-any prec7
match precedence 7
end-class-map
!
class-map match-any prec6
match precedence 6
end-class-map
!
class-map match-any prec5
match precedence 5
end-class-map
!
class-map match-any prec4
match precedence 4
end-class-map
!
class-map match-any prec3
match precedence 3
end-class-map
!
class-map match-any prec2
match precedence 2
end-class-map
!
class-map match-any prec1
match precedence 1
end-class-map
!
!
policy-map QOS_MARKING
class prec7
set traffic-class 7
set qos-group 7
!
class prec6
set traffic-class 6
set qos-group 6
!
class prec5
set traffic-class 5
set qos-group 5
!
class prec4
set traffic-class 4
set qos-group 4
!
class prec3
set traffic-class 3
set qos-group 3
!
class prec2
set traffic-class 2
set qos-group 2
!
class prec1
set traffic-class 1
set qos-group 1
!
class class-default
set traffic-class 0
set qos-group 0
!
*Egress:*
class-map match-any tc7
match traffic-class 7
end-class-map
!
class-map match-any tc6
match traffic-class 6
end-class-map
!
class-map match-any tc5
match traffic-class 5
end-class-map
!
class-map match-any tc4
match traffic-class 4
end-class-map
!
class-map match-any tc3
match traffic-class 3
end-class-map
!
class-map match-any tc2
match traffic-class 2
end-class-map
!
class-map match-any tc1
match traffic-class 1
end-class-map
!
policy-map QOS_QUEUING
class tc7
priority level 1
shape average percent 10
!
class tc6
bandwidth remaining ratio 1
queue-limit 100 ms
!
class tc5
bandwidth remaining ratio 20
queue-limit 100 ms
!
class tc4
bandwidth remaining ratio 20
random-detect ecn
random-detect 6144 bytes 100 mbytes
!
class tc3
bandwidth remaining ratio 20
random-detect ecn
random-detect 6144 bytes 100 mbytes
!
class tc2
bandwidth remaining ratio 5
queue-limit 100 ms
!
class tc1
bandwidth remaining ratio 5
queue-limit 100 ms
!
class class-default
bandwidth remaining ratio 20
queue-limit 100 ms
!
[buffer-extended]
hw-module profile priority-flow-control location 0/0/CPU0
buffer-extended traffic-class 3 pause-threshold 10 ms
buffer-extended traffic-class 4 pause-threshold 10 ms
!
[buffer-internal]
hw-module profile priority-flow-control location 0/1/CPU0
buffer-internal traffic-class 3 pause-threshold 403200 bytes headroom 441600 bytes ecn 224640 bytes
buffer-internal traffic-class 4 pause-threshold 403200 bytes headroom 441600 bytes ecn 224640 bytes
Verification
Router#sh controllers hundredGigE0/0/0/22 priority-flow-control
Priority flow control information for interface HundredGigE0/0/0/22:
Priority Flow Control:
Total Rx PFC Frames : 0
Total Tx PFC Frames : 313866
Rx Data Frames Dropped: 0
CoS Status Rx Frames
--- ------ ----------
0 on 0
1 on 0
2 on 0
3 on 0
4 on 0
5 on 0
6 on 0
7 on 0
/*[buffer-internal]*/
Router#show controllers hundredGigE 0/9/0/24 priority-flow-control
Priority flow control information for interface HundredGigE0/9/0/24:
Priority Flow Control:
Total Rx PFC Frames : 0
Total Tx PFC Frames : 313866
Rx Data Frames Dropped: 0
CoS Status Rx Frames
--- ------ ----------
0 on 0
1 on 0
2 on 0
3 on 0
4 on 0
5 on 0
6 on 0
7 on 0
…
/*[buffer-internal, tc3 & tc4 configured. TC4 doesn’t have ECN]*/
Router#show controllers npu priority-flow-control location <loc>
Location Id: 0/1/CPU0
PFC: Enabled
PFC-Mode: buffer-internal
TC Pause Headroom ECN
-------------------------------------------------------
3 86800 bytes 120000 bytes 76800 bytes
4 86800 bytes 120000 bytes Not-configured
/*[buffer-extended PFC, tc3 & tc4 configured]*/
Router#show controllers npu priority-flow-control location <loc>
Location Id: 0/1/CPU0
PFC: Enabled
PFC-Mode: buffer-extended
TC Pause
-----------
3 5000 us
4 10000 us
/*[No PFC]*/
Router#show controllers npu priority-flow-control location <loc>
Location Id: 0/1/CPU0
PFC: Disabled
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