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

buffer-internal mode

Updated: September 11, 2026

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Details buffer-internal mode for PFC, including pause-threshold, headroom, and ECN threshold configuration in the line card profile, along with best practices and reload requirements.

A buffer-internal mode is a Priority Flow Control (PFC) operating mode that

  • configures pause-threshold, headroom, and ECN thresholds in the line card profile

  • calculates the effective queue limit as the sum of pause-threshold and headroom, and

  • applies uniformly to all ports on a line card.

Supporting details for buffer-internal mode

  • Explicit Congestion Notification (ECN) and queue-limit values in interface queuing policies are ignored when buffer-internal mode is active.

  • Recommended for short-haul PFC peers (less than 1 km apart) because thresholds are enforced close to the hardware buffers.

  • Provides predictable, lossless behavior for designated traffic classes.

Lane traffic analogy for buffer-internal mode

Each lane has a small local holding bay. As the bay fills, a yellow warning line (ECN threshold) signals drivers to ease off. If it continues filling to the red stop line (pause threshold), a stop sign is raised for that lane. Extra reserved space beyond the stop line (headroom) catches cars already en route while the stop takes effect.

Best practices for configuring buffer-internal mode

buffer-internal mode threshold ranges

Use this information to confirm valid threshold ranges and relationships for buffer-internal mode.

  • Pause threshold values: 307,200 to 422,400 bytes.

  • Headroom threshold values: 345,600 to 537,600 bytes.

  • Explicit Congestion Notification (ECN) threshold values: 153,600 to 403,200 bytes.

Guidelines to configure buffer-internal mode

Follow these guidelines to ensure a stable Priority Flow Control (PFC) configuration in the buffer-internal mode.

  • For Cisco 8808 and Cisco 8812 chassis, configure Priority Flow Control (PFC) thresholds on all line cards in the chassis.

  • Reload the line card whenever you add or remove a traffic class. You must also reload the line card when you configure the buffer-internal threshold values for the first time on a new traffic class.

  • Add or remove ECN configuration using the hw-module profile priority-flow-control command. Then, reload the line card to apply the ECN changes.

Restrictions while configuring buffer-internal mode

Enforce these restrictions while configuring the buffer-internal mode thresholds.

  • Ensure the ECN threshold value is less than the pause threshold value.

  • Ensure that the combined configuration values for pause threshold and headroom do not exceed 844,800 bytes; otherwise, the configuration is rejected.

  • Configure only one of buffer-internal, buffer-extended, or buffer-extended hybrid mode on each line card.

Configure buffer-internal thresholds

Use this task to configure pause-threshold, headroom, and Explicit Congestion Notification (ECN) thresholds for traffic classes in buffer-internal mode.

  • Applies uniformly to all interfaces on the line card.

  • Use only when buffer-extended or buffer-extended hybrid mode is not configured on the same line card.

Before you begin

Ensure that peer devices are also configured for Priority Flow Control on the same class of service (CoS).

Follow these steps to configure buffer-internal thresholds.

Procedure

  1. Enter the line card profile configuration.

    Example:

    Router(config)#hw-module profile priority-flow-control mode buffer-internal

    This example selects buffer-internal mode for PFC on the line card.

  2. Configure thresholds per traffic class.

    Example:

    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

    Each value in this example is described here.

    • traffic-class 3 : Selects traffic class 3 out of 0–7. This TC is tied to a Layer 2 CoS (802.1p) or an internal mapping, so your QoS policy should map the desired CoS or DSCP to traffic class 3 if you want this class to be lossless.

    • pause-threshold 307200 : When instantaneous buffer occupancy for TC 3 reaches 307,200 bytes, the device starts sending PFC pause frames for priority 3. This prevents loss by pausing upstream senders before the queue overflows.

    • headroom 345600 : This is the reserved buffer for traffic class 3 to absorb traffic after a pause is asserted. It's the guaranteed space the queue can grow into while the pause takes effect.

    • ecn-threshold 153600 : Explicit Congestion Notification (ECN) marking begins once occupancy exceeds 153,600 bytes. This value should be lower than the pause threshold so congestion is signaled (via ECN) before resorting to PFC pauses.

  3. (Optional) Configure ECN maximum threshold and probability percentage (Pmax).

    Example:

    Router(config-pfc-profile-tc)#ecn-max-threshold 403200
Router(config-pfc-profile-tc)#probability-percentage 5
Router(config-pfc-profile-tc)#exit
Router(config-pfc-profile)#exit
Router(config)#commit

    Here's what each value indicates in this example.

    • ecn-max-threshold 403200 : Upper bound of the ECN marking ramp. Between ecn-threshold and ecn-max-threshold , marking probability increases linearly. At or above this level, the marking probability reaches the configured maximum percentage (Pmax).

    • probability-percentage 5 : Indicates 5% as the maximum ECN marking probability (Pmax) at the ecn-max-threshold value. In effect, this means that there will be 0% marking at 153,600 bytes, linearly increasing to 5% at 403,200 bytes and above.

  4. View the configured thresholds and runtime counters for the specific line card.

    Example:

    Router#show controllers npu priority-flow-control location 0/0/CPU0
TC  Pause-threshold  Headroom  ECN-threshold  ECN-max  Pmax  TxPause
3   307200           345600    153600         403200   5%    24

    Each value in this example is described here.

    • TC : Traffic Class (3)

    • Pause-threshold : 307,200 bytes (pause onset).

    • Headroom : 345,600 bytes (reserved buffer after pause).

    • ECN-threshold : 153,600 bytes (start of ECN marking).

    • ECN-max : 403,200 bytes (top of ECN ramp).

    • Pmax : 5% (maximum ECN marking probability).

    • TxPause : 24 (number of PFC pause frames transmitted for this TC).