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

PDF

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

buffer-extended mode

Updated: June 9, 2026

Want to summarize with AI?

Log in

Details buffer-extended mode for PFC, including pause-threshold configuration, interface-level ECN and queue limits, long-haul peer recommendations, and threshold configuration guidelines.

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

  • allows pause-thresholds to be configured in the line card profile

  • requires Explicit Notification Congestion (ECN) and queue limit configuration in the interface’s egress queuing policies, and

  • applies consistently to all ports on the line card.

Supporting details for buffer-extended mode

  • Provides more flexibility than buffer-internal mode, since ECN and queue limits can differ per interface.

  • Recommended for long-haul PFC peers, such as data center interconnects over 1 km.

  • Queue-limit and ECN configurations in policies take effect, unlike buffer-internal mode where they are ignored.

Lane traffic analogy for buffer-extended mode

Each lane still has its own small holding bay, but the city traffic authority sets one common stop line for everyone. This is the pause-threshold defined in the line-card profile.

Each lane’s local signal controller (the interface-level QoS policy) decides when to flash its yellow warning light, which is the ECN threshold that each interface can tune separately.

So, while all lanes share the same stop rule, each lane can warn drivers earlier or later depending on how busy it gets.

Best practices for configuring buffer-extended mode

buffer-extended mode threshold ranges

Set the pause-threshold value range for buffer-extended mode between 2 milliseconds (ms) and 25 ms or from 2000 microseconds (μs) and 25000 μs.

Guidelines to configure buffer-extended mode

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

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

  • For line card 88-LC0-36FH-M, use kilobytes (KB) or megabytes (MB) for both pause and headroom thresholds. The headroom value range is 4 to 75,000.

  • For line card 8800-LC-48H, use milliseconds (ms) or microseconds (μs) for pause thresholds. Do not configure headroom values. Always use ms or μs even if KB or MB are shown in the CLI.

Restrictions while configuring buffer-extended mode

Enforce these restrictions while configuring the buffer-extended 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-extended thresholds

Use this task to configure Priority Flow Control (PFC) pause-thresholds in the line-card profile and define Explicit Congestion Notification (ECN) and queue-limit parameters in interface-level QoS policies.

buffer-extended mode separates global pause configuration from per interface congestion control:

  • Pause thresholds are configured once in the line-card profile.

  • ECN and queue limits are set per interface in QoS policies.

This enables fine-grained traffic management across multiple interfaces on the same line card.

Before you begin

  • The router must already be configured for buffer-extended mode.

  • PFC must be globally enabled (priority-flow-control mode on ).

Follow these steps to configure buffer-extended thresholds.

Procedure

1.

Enter the line card PFC profile and enable buffer-extended mode.

Example:

Router(config)#hw-module profile priority-flow-control mode buffer-extended
2.

Specify the pause-threshold value for the desired traffic class.

Example:

Router(config-pfc-profile)#traffic-class 3
Router(config-pfc-profile-tc)#pause-threshold 2000 us

  • All pause-threshold values configured under this profile will apply uniformly to every port on that line card.

  • This step defines the point at which PFC sends a pause frame upstream for traffic class 3 when congestion is detected.

  • The unit (μs, ms, KB, or MB) depends on your specific Cisco 8000 router, but the function remains the same: this value triggers pausing traffic on the specified class of service (CoS).

3.

(Optional) Configure additional buffer parameters if supported by your line card.

Example:

Router(config-pfc-profile-tc)#headroom 600 KB

  • On some Cisco 8000 routers, you can explicitly reserve additional buffer space beyond the pause threshold (called headroom) to absorb packets already in flight when a pause frame is issued.

  • On hardware where headroom is not configurable, this step is omitted.
4.

Create or edit the interface-level QoS policy to define ECN thresholds.

Example:

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

  • This step creates or edits the interface-level QoS policy that defines ECN thresholds and probabilities.

  • The random-detect command enables Weighted Random Early Detection (WRED), the mechanism that provides early congestion signaling before a PFC pause frame is triggered.

  • The min-threshold (200 KB) marks the queue depth where ECN marking begins.

  • The max-threshold (400 KB) is the queue depth where ECN marking probability increases to the configured probability (5 %).

  • Between these two points, the system automatically increases the marking probability in a linear fashion.

  • If ECN is enabled, packets are marked with ECN bits instead of being dropped.

  • If ECN is disabled, packet drops occur early to prevent queue buildup.

This configuration allows the router to signal congestion early, reducing the likelihood of abrupt PFC pauses and maintaining smoother traffic flow.
5.

Apply the QoS policy to the required interface.

Example:

Router(config)#interface HundredGigE0/0/0/2
Router(config-if)#service-policy output qos-policy
Router(config)#exit
Router(config)#commit

  • This step creates or edits the interface-level QoS policy that defines ECN thresholds and probabilities.

  • The random-detect command enables Weighted Random Early Detection (WRED), the mechanism that provides early congestion signaling before a PFC pause frame is triggered.

  • The min-threshold (200 KB) marks the queue depth where ECN marking begins.

  • The max-threshold (400 KB) is the queue depth where ECN marking probability increases to the configured probability (5%).

  • Between these two points, the system automatically increases the marking probability in a linear fashion.

  • If ECN is enabled, packets are marked with ECN bits instead of being dropped.

  • If ECN is disabled, packet drops occur early to prevent queue buildup.

This configuration allows the router to signal congestion early, reducing the likelihood of abrupt PFC pauses and maintaining smoother traffic flow.
6.

View pause thresholds applied on the line card.

Example:

Router#show controllers npu priority-flow-control location 0/1/CPU0
Traffic Class   Pause-Threshold   ECN Source   Status
3               2000 us           Policy-Map   Enabled

  • The configured traffic class (3) now uses a 2000 μs pause threshold.

  • The ECN source is listed as Policy-Map , confirming that ECN thresholds are coming from interface-level configuration.

  • The min-threshold (200 KB) marks the queue depth where ECN marking begins.

  • The max-threshold (400 KB) is the queue depth where ECN marking probability increases to the configured probability (5 %).

  • Between these two points, the system automatically increases the marking probability in a linear fashion.

  • If ECN is enabled, packets are marked with ECN bits instead of being dropped.

  • If ECN is disabled, packet drops occur early to prevent queue buildup.

This configuration allows the router to signal congestion early, reducing the likelihood of abrupt PFC pauses and maintaining smoother traffic flow.
7.

View ECN thresholds and queue limits on the interface.

Example:

Router#show policy-map interface HundredGigE0/0/0/2
Class-map: COS_3
  Random Detect (WRED):
    Min Threshold : 200 KB
    Max Threshold : 400 KB
    Probability   : 5 %
    Mark Type     : ECN

  • The configured ECN minimum and maximum thresholds are applied to the interface.

  • Mark Type : ECN confirms that early congestion notification is active instead of packet drops.

  • The min-threshold (200 KB) marks the queue depth where ECN marking begins.

  • The max-threshold (400 KB) is the queue depth where ECN marking probability increases to the configured probability (5 %).

  • Between these two points, the system automatically increases the marking probability in a linear fashion.

  • If ECN is enabled, packets are marked with ECN bits instead of being dropped.

  • If ECN is disabled, packet drops occur early to prevent queue buildup.

This configuration allows the router to signal congestion early, reducing the likelihood of abrupt PFC pauses and maintaining smoother traffic flow.