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

Queue buffer optimization based on average packet size

Updated: September 11, 2026

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Explains how the NPU average-packet-size profile increases usable queue buffering for burst absorption and for PFC deployments that require high pause thresholds and headroom.

Queue buffer optimization based on average packet size is a QoS buffering capability that

allows a queue to use higher buffer capacity based on the configured average packet size,
increases burst absorption before packet drops in non-PFC deployments, and
helps prevent queue drops in PFC deployments that use high pause thresholds and headroom.

Non-PFC deployments—Use the profile to allow higher maximum buffering per queue for the configured average packet size. The additional buffering increases the amount of traffic burst that a queue can absorb before packet drops occur.

PFC deployments—Use the profile for deployments that require high pause thresholds and headroom. The additional usable queue buffering helps prevent packet drops while PFC controls upstream traffic.

Data Center Interconnect deployments—For PFC-based Data Center Interconnect (DCIX) roles, the profile supports long links up to 120 km where high pause thresholds and headroom are required to prevent packet drops.

Table 1. Feature History Table
Feature Name	Release Information	Feature Description
Queue buffer optimization based on average packet size	Release 26.3.1	Introduced in this release on: Fixed Systems (8200 [ASIC: Q200]); Centralized Systems (8600 [ASIC: Q200]); Modular Systems (8800 [LC ASIC: Q200]) You can now optimize the maximum buffering available to a queue for a configured average packet size. This increases burst absorption before packet drops in non-PFC deployments and helps prevent queue drops in PFC deployments that use high pause thresholds and headroom, including Data Center Interconnect (DCI) links up to 120 km. The feature enables this behavior by configuring the NPU average-packet-size profile and reloading the router to apply the profile.

Benefits of queue buffer optimization based on average packet size

Queue buffer optimization based on average packet size provides these benefits:

Higher burst absorption in non-PFC deployments—Allows a queue to use higher buffer capacity for the configured average packet size, enabling it to absorb larger traffic bursts before packet drops occur.
Reduced queue drops in PFC deployments—Provides additional usable queue buffering for deployments that require high pause thresholds and headroom, helping prevent packet drops while PFC controls upstream traffic.
Long-distance PFC deployment support—Provides the additional buffering required for PFC-based deployment scenarios, such as Data Center Interconnect roles over links up to 120 km that use high pause thresholds and headroom.
Supports jumbo frames beyond 4K over long links—Enables deployments to use jumbo frames larger than 4K across long-distance links, extending the buffering capability for higher packet-size traffic without relying on a 4K MTU limit.

Usage guidelines for queue buffer optimization

High-buffering requirements

Configure the average-packet-size profile when a queue must use higher buffer capacity for the expected average packet size.

In non-PFC deployments, use the additional buffering to increase burst absorption before packet drops.
In PFC deployments, use the additional buffering for high pause threshold and headroom use cases that must prevent queue drops.

Long-distance PFC deployments

Use the profile for Long-distance PFC deployments, such as PFC-based Data Center Interconnect (DCI) deployments over links up to 120 km when high pause thresholds and headroom are required to prevent queue drops.

Configure the average packet size profile

Use this task to configure the average packet size that the NPU uses for queue buffer optimization.

Before you begin

Complete these prerequisites before you begin:

Determine the average packet size value required for your deployment.
Plan a router reload after you configure the profile.
For PFC deployments, determine whether the deployment requires high pause thresholds and headroom.

Procedure

Configure the NPU average-packet-size profile.

Example:

Router(config)# hw-module profile qos hbm-average-pkt-size 4178
Router(config)# commit

Reload the router to apply the profile.
Example:
```
Router# reload location all
```

Verify that the profile is configured and applied.

Example:

Router# show hw-module hbm-average-pkt-size

Location       Configured     Applied          Action
------------------------------------------------------------
0/6/CPU0       Yes            Yes              N/A
0/4/CPU0       Yes            Yes              N/A

The command shows whether the profile is configured and applied and whether further action is required.

The configured average-packet-size profile is applied to the NPU after the router reloads. The verification output shows the profile state for each location.

Queue buffer optimization details

Use this information to identify the expected buffering outcome for each deployment type and interpret the profile verification output.

Table 2. Queue buffer optimization use cases
Deployment	Buffering requirement	Result
Non-PFC	Higher maximum buffering per queue for the configured average packet size	Increases burst absorption before packet drops.
PFC	High pause threshold and headroom	Helps prevent queue drops.

NPU average packet size profile verification

Use the show hw-module hbm-average-pkt-size command to check the current profile state.

Table 3. Verification output fields
Field	Description
Configured	Indicates whether the average-packet-size profile is configured for the location.
Applied	Indicates whether the configured profile is applied for the location.
Action	Indicates whether further action, such as a reload, is required.

RP/0/RP0/CPU0:R1# show hw-module hbm-average-pkt-size

Location       Configured     Applied          Action
------------------------------------------------------------
0/6/CPU0       Yes            Yes              N/A
0/4/CPU0       Yes            Yes              N/A

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