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

Low-latency queuing with strict priority queuing

Updated: June 9, 2026

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Details LLQ with strict priority queuing for prioritizing delay-sensitive traffic, including configuration guidelines, priority levels, shaping options, and verification of egress queue behavior.

Low-latency queuing with strict priority queuing is a QoS congestion management feature that

  • integrates strict priority queuing into the Class-Based Weighted Fair Queuing (CBWFQ) model

  • ensures delay-sensitive traffic is always dequeued ahead of other traffic classes, and

  • minimizes latency and jitter for critical applications such as voice and real-time video.

CBWFQ is a QoS scheduling model that assigns bandwidth to traffic classes based on configured weights and ensures each class receives its allocated portion of service during congestion.

Priority Queuing is a scheduling method that dequeues packets from the highest-priority queue before any other queue, allowing delay-sensitive traffic to pre-empt lower-priority classes; in strict priority mode, lower-priority queues may not be serviced if high-priority traffic is continuously present.

Table 1. Feature History Table

Feature Name

Release Information

Feature Description

Priority Propagation on Egress Queues

Release 26.1.1

Introduced in this release on: Centralized Systems (8400 [ASIC: K100])(select variants only*)

* This feature is now supported on Cisco 8404-SYS-D routers.

Priority Propagation on Egress Queues

 Release 25.4.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100]) (select variants only*)

*This feature is supported on:

  • 8711-48Z-M

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Priority Propagation on Egress Queues

 Release 25.1.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100])(select variants only*)

This feature is supported on:

  • 8712-MOD-M

  • 8011-4G24Y4H-I

Low Latency Queueing with Strict Priority Queueing

 Release 25.4.1

Introduced in this release on: Fixed Systems (8700 [ASIC: K100], 8010 [ASIC: A100]) (select variants only*)

*This feature is supported on:

  • 8011-32Y8L2H2FH

  • 8011-12G12X4Y-A/D

Low Latency Queueing with Strict Priority Queueing

 Release 25.1.1

Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*)

*This feature is supported on Cisco 8011-4G24Y4H-I routers.

Low Latency Queueing with Strict Priority Queueing

 Release 24.4.1

Introduced in this release on: Fixed Systems (8700) (select variants only*)

This feature ensures that time-sensitive traffic receives dedicated priority, ensuring minimal latency and jitter while congestion for other types of traffic is managed fairly, guaranteeing high performance for critical applications and efficient overall traffic management.

*This functionality is now supported on Cisco 8712-MOD-M routers.

Priority Propagation on Egress Queues

 Release 24.3.1

Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100])(select variants only*); Modular Systems (8800 [LC ASIC: P100])(select variants only*)

You can now ensure that high-priority traffic is consistently prioritized across multiple sub-interfaces on the same network port, enhancing the performance of critical applications.

This is achieved by enabling support for egress queuing policy maps that allow high-priority P1 traffic (on class TC7) to take precedence over non-P1 traffic across sub-interfaces. This feature ensures that critical traffic, such as voice or real-time video, is always given the highest priority, regardless of the sub-interface it is associated with, thereby maintaining optimal network performance and reducing latency for essential services.

*This feature is supported on:

  • 8212-48FH-M

  • 8711-32FH-M

  • 88-LC1-36EH

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

Low Latency Queueing with Strict Priority Queueing

 Release 24.3.1

Introduced in this release on: Modular Systems (8800 [LC ASIC: P100]) (select variants only*), Fixed Systems (8200) (select variants only*), Fixed Systems (8700 (P100, K100)) (select variants only*)

This feature ensures that time-sensitive traffic receives dedicated priority, ensuring minimal latency and jitter while congestion for other types of traffic is managed fairly, guaranteeing high performance for critical applications and efficient overall traffic management.

*This feature is supported on:

  • 88-LC1-12TH24FH-E

  • 88-LC1-52Y8H-EM

  • 8212-48FH-M

  • 8711-32FH-M

Low Latency Queueing with Strict Priority Queueing

 Release 24.2.11

Introduced in this release on: Modular Systems (8800 [LC ASIC: P100])(select variants only*)

With this feature, time-sensitive traffic receives dedicated priority, ensuring minimal latency and jitter while congestion for other types of traffic is managed fairly, guaranteeing high performance for critical applications and efficient overall traffic management.

You can also configure low latency queueing with strict priority queueing at the subinterface level, ensuring that the strict priority policy applies only to traffic on that specific subinterface, not the entire physical interface. This configuration is useful in scenarios where multiple services share the same physical link but have different QoS requirements.

*This feature is supported on 88-LC1-36EH.

Guidelines for configuring low-latency queuing with strict priority queuing

Priority and scheduling behavior

  • Only priority levels 1 to 7 are supported, where 1 is the highest and 7 is the lowest priority; the default CoSQ 0 has the lowest priority among all classes.

  • Egress policing is not supported, and in strict priority mode, heavy priority traffic can prevent lower-priority queues from being serviced.

  • The platform enforces a strict class hierarchy, where higher traffic classes take precedence over lower ones. This hierarchy can be redefined through an egress queuing policy map when configuring LLQ with strict priority.

  • When operating in strict-priority mode, egress scheduling follows the default class order TC7 >TC6 > TC5 > TC4 >TC3 > TC2 > TC1 > TC0, unless a queuing policy map overrides this behavior.

Shaping and rate control

  • You can configure shape average and queue-limit commands along with the priority command.

  • An optional shaper can be configured to cap the maximum traffic rate and prevent starvation of lower-priority traffic classes.

Interface-level configuration

LLQ with strict priority queuing can also be applied at the subinterface level, allowing strict priority behavior to affect only the specific subinterface rather than the entire physical interface.

Configure low-latency queuing with strict priority queuing

Before you begin

Ensure that you have an existing QoS class map that classifies the intended traffic.

Follow these steps to configure LLQ with strict priority on an egress interface.

Procedure

1.

Create or modify a policy-map that applies LLQ with strict priority.

This policy-map defines the class of traffic that receives strict priority handling.

Example:


            Router# configure
            Router(config)# policy-map test-priority-1
            Router(config-pmap)# class qos1
            Router(config-pmap-c)# priority level 7
            Router(config-pmap-c)# exit
            Router(config-pmap)# exit

The policy-map now includes a strict priority class with the specified priority level.
2.

(Optional) Shape the strict priority class to a specific bit rate.

Shaping prevents lower-priority queues from starving during high volumes of strict priority traffic.

Example:

Router(config-pmap-c)# shape average percent 40
3.

Apply the policy-map to the egress interface.

Attaching the policy-map makes strict priority queuing active for egress traffic on the specified interface.

Example:

Router(config)# interface HundredGigE 0/6/0/18
Router(config-if)# service-policy output test-priority-1
Router(config-if)# no shutdown
Router(config-if)# commit

The LLQ strict priority policy is now active on the selected output interface.
4.

Verify the strict priority behavior on the interface.

Confirm that the strict priority class (for example, HP7) is visible in the queue statistics.

Example:


            Router# show qos interface HundredGigE 0/6/0/18 output
            
            NOTE:- Configured values are displayed within parentheses
            Interface HundredGigE0/6/0/18 ifh 0x3000220  -- output policy
            NPU Id:                        3
            Total number of classes:       3
            Interface Bandwidth:           100000000 kbps
            VOQ Base:                      11176
            VOQ Stats Handle:              0x88550ea0
            Accounting Type:               Layer1 (Include Layer 1 encapsulation and above)
            ------------------------------------------------------------------------------
            Level1 Class (HP7)                       =   qos1
            Egressq Queue ID                         =   11177 (HP7 queue)
            TailDrop Threshold                       =   125304832 bytes / 10 ms (default)
            WRED not configured for this class
            
            Level1 Class (HP6)                       =   qos-2
            Egressq Queue ID                         =   11178 (HP6 queue)
            TailDrop Threshold                       =   125304832 bytes / 10 ms (default)
            WRED not configured for this class
            
            Level1 Class (class-default)
            Egressq Queue ID                         =   11176 (Default LP queue)
            Queue Max. BW.                           =   101803495 kbps (default)
            Queue Min. BW.                           =   0 kbps (default)
            Inverse Weight / Weight                  =   1 (BWR not configured)
            TailDrop Threshold                       =   1253376 bytes / 10 ms (default)
            WRED not configured for this class

The interface shows the strict priority class and confirms that LLQ with strict priority is functioning as expected.