Modular QoS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Release

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Modular QoS Configuration Guide for Cisco 8000 Series Routers, Cisco IOS XR Release

Configure PFC on interfaces

Updated: September 11, 2026

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Provides instructions for enabling PFC on interfaces, including class-based pause behavior, policy requirements, interface configuration, and verification of PFC operation.

Use this task to enable Priority Flow Control (PFC) on supported line cards and interfaces, and to apply the classification and queuing policies that allow selected traffic classes to operate losslessly.

  • PFC must be enabled globally and per line card before it can be applied to traffic classes.

  • The operating mode (buffer-internal, buffer-extended, or buffer-extended hybrid) must be chosen at the line card profile level.

Before you begin

  • Install the supported line card.

  • Configure PFC on matching CoS values.

  • Ensure there are no breakout, bundle, or sub-interface configurations on PFC-enabled line cards.

Follow these steps to configure PFC on an interface.

Procedure

  1. Enable PFC globally.

    Example:

    Router(config)#priority-flow-control mode on

  2. Configure the line card profile for the required PFC mode.

    Example:

    Router(config)#priority-flow-control mode on
    • Option A: buffer-internal mode 
       Router(config)#hw-module profile priority-flow-control mode buffer-internal
Router(config-pfc-profile)#traffic-class 3
Router(config-pfc-profile-tc)#pause-threshold 307200
Router(config-pfc-profile-tc)#headroom 345600
Router(config-pfc-profile-tc)#ecn-threshold 153600

      This option defines all threshold values directly in the LC profile so each traffic class uses fixed on-chip buffers.

    • Option B: buffer-extended mode 
      Router(config)#hw-module profile priority-flow-control mode buffer-extended
Router(config-pfc-profile)#traffic-class 3
Router(config-pfc-profile-tc)#pause-threshold 2000 us

      • This option configures only the global pause-threshold in the line card profile while leaving Explicit Congestion Notification (ECN) and queue thresholds to interface policies.

      • This option centralizes pause control across the line card but allows per-interface tuning through QoS policies.

    • Option C: buffer-extended hybrid 
      Router(config)#hw-module profile priority-flow-control hybrid-mode enable
Router(config-pfc-profile)#hbm-buffers-percentage lossless 60 lossy 40
Router(config-pfc-profile)#max-non-pfc-voqs 2500

      This option divides High Bandwidth Memory (HBM) between lossless and lossy traffic:

      • hbm-buffers-percentage allocates 60 percent of HBM to the lossless pool and 40 percent to lossy traffic.

      • max-non-pfc-voqs caps the number of lossy queues that share HBM resources.

      • The configuration takes effect after a router restart.

  3. Configure and attach classification and queuing policies.

    Example:

    Router(config)#policy-map pfc-ingress
Router(config-pmap)#class COS_3
Router(config-pmap-c)#priority-flow-control enable
Router(config)#policy-map qos-policy
Router(config-pmap)#class COS_3
Router(config-pmap-c)#random-detect min-threshold 200 KB max-threshold 400 KB probability 5
Router(config-pmap-c)#random-detect ecn
Router(config-pmap-c)#exit
Router(config-pmap)#exit
Router(config)#interface HundredGigE0/0/0/1
Router(config-if)#service-policy input pfc-ingress
Router(config-if)#service-policy output qos-policy
Router(config)#commit

    This example:

    • creates a policy map named pfc-ingress to handle incoming traffic related to PFC.

    • defines a class named COS_3 inside the pfc-ingress policy map.

    • enables PFC processing on incoming traffic for COS_3 .

    • creates another policy map named qos-policy to apply QoS mechanisms to outgoing traffic.

    • defines a class named COS_3 within the qos-policy policy map.

    • configures Random Early Detection (RED), min-threshold , max-threshold , and the probability of dropping packets for COS_3 traffic.

    • enables Explicit Congestion Notification (ECN) in conjunction with RED.

    • applies the pfc-ingress policy map to the input direction of the HundredGigE0/0/0/1 interface. This means that any traffic arriving on this interface will be evaluated against the pfc-ingress policy, and the router will honor incoming PFC pause frames for COS_3 traffic.

    • applies the qos-policy policy map to the output direction of the HundredGigE0/0/0/1 interface. This means that any traffic being transmitted out of this interface will be evaluated against the qos-policy , and the RED or ECN mechanisms will be applied to COS_3 traffic to manage congestion.

  4. Verify the hardware-level PFC profile configuration on a specific line card.

    Example:

    Router#show controllers npu priority-flow-control location 0/0/CPU0
Location: 0/0/CPU0
PFC Mode: buffer-internal

Traffic Class  Pause-threshold  Headroom  ECN-threshold  Tx Pause Frames
-------------  ---------------  --------  -------------  ---------------
3              307200           345600    153600         24

    A non-zero and increasing Tx Pause Frames count indicates that the router's NPU is actively experiencing congestion for this traffic class and is sending pause frames to its neighbors. This is a key indicator that PFC is working as intended.

  5. View the PFC statistics for a specific interface.

    Example:

    Router#show controllers HundredGigE0/0/0/1 priority-flow-control
Priority flow control information for interface HundredGigE0/0/0/1:
Priority Flow Control:
Total Rx PFC Frames : 95
Total Tx PFC Frames : 102
Rx Data Frames Dropped: 0
CoS Status Rx Frames
--- ------ ----------
0   on     0
1   on     0
2   on     0
3   on     95
4   on     0
5   on     0
6   on     0
7   on     0

    • Total Rx PFC Frames : 95 indicates that this interface has received 95 PFC pause frames in total.

    • Total Tx PFC Frames : 102 indicates that this interface has transmitted 102 PFC pause frames in total.

    • Rx Data Frames Dropped: 0 indicates that the PFC mechanism is effectively preventing data frame drops on ingress due to congestion on this interface.

  6. To view the consolidated statistics for all interfaces on the router, see View global statistics for Priority Flow Control and Priority Flow Control watchdog.