Cisco QoS Evolution: Hierarchical Queueing Framework

Ed. note: This post assumes that you are already familiar with the previous class-map, policy-map, and service-policy options taught in the Cisco QoS class.
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Cisco IOS 12.4(20T) introduced a new QoS paradigm titled the Hierarchical Queueing Framework (HQF). The hierarchical queueing framework uses the same modular QoS CLI (MQC) framework that was introduced way back in IOS 12.1(5T), but the HQF model has many enhancements that we will take a look at in this post.

Modular platforms like the 10000, 7500, and 12XXX routers support distributed Cisco express forwarding (dCEF) technology that allows forwarding information bases (FIB) and QoS operations to be pushed down to the line card and processed without interrupting the route processor and traversing the backplane of the router. These modular platforms require very specific command syntax to push the operation down to the line card, however. If QoS operations are not performed on the line cards in these platforms, there may be a very detrimental impact to the CPU of the route processor. The HQF provides a consistent QoS approach across both non-distributed and distributed platforms.

Hierarchical traffic shaping policies have been supported in the MQC for a long time. The HQF takes the hierarchical nature to the next level by allowing nested policy-maps that both provide bandwidth guarantees.

The HQF also promises to have more consistent behavior across different router platforms. Older versions of the MQC only allowed 75% of the interface bandwidth to be assigned to a policy. The HQF allows 100% of the interface bandwidth to be assigned and provides a minimum of 1% bandwidth to the class-default.

Policy-maps can now be applied to generic route encapsulation (GRE) tunnels and the bandwidth aggregate of logical interfaces (usually sub-interfaces) can exceed the aggregate bandwidth available at the main interface.

References
Hierarchical Queueing Framework (HQF) – IOS 12.4(20T) QoS Features

Author: Dennis Hartmann

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