QoS - Nested policy maps

In the context of Quality of Service, nested policy maps are an advanced configuration technique used to apply multiple layers of QoS policies to network traffic. This hierarchical approach to QoS allows for more granular and sophisticated traffic management. Here's a breakdown of how it works:

Concept of Nesting

  1. Basic Policy Map: A policy map is a collection of QoS settings applied to traffic. It typically includes class maps (which classify traffic based on criteria like IP address, port number, etc.) and defines actions (like prioritizing, shaping, or policing traffic).
  2. Nested Policy Maps: In a nested structure, you have a parent policy map and one or more child policy maps. The child policy maps are referenced within the parent policy map. This nesting allows for the application of multiple QoS policies to the same traffic stream, each at different levels of granularity.

Application and Advantages

  1. Hierarchical QoS (HQoS): Nested policy maps are often used in hierarchical QoS scenarios. For instance, on a WAN interface, you might have a parent policy for overall bandwidth management and child policies for managing different types of traffic like VoIP, video, and data.
  2. Flexibility and Precision: By nesting policies, network administrators can apply very specific QoS treatments to different types of traffic. This is especially useful in complex networks with diverse traffic types and requirements.
  3. Efficiency: Nested policy maps help in efficiently utilizing bandwidth and ensuring that critical applications get the necessary resources while less important traffic is contained within defined limits.

Implementation Considerations

  1. Processing Order: It's important to understand the order in which policies are applied. The child policy maps are processed first, followed by the parent policy map.
  2. Resource Intensiveness: Nested policy maps can be resource-intensive. They require careful planning and testing to ensure they don’t overwhelm router resources.
  3. Compatibility and Limitations: Not all Cisco devices support nested policy maps, and there may be limitations based on the device model and IOS version.

Use Cases

  • Enterprise WAN Links: Managing different application types traversing a WAN link.
  • Service Provider Networks: Offering differentiated services to various customer segments.
  • Campus Networks: Prioritizing critical services like voice and video over general data traffic.

Nested policy maps are a powerful tool in the QoS toolkit for network engineers, offering the ability to finely tune network performance and resource allocation. However, they require a good understanding of both the network’s needs and the capabilities of the Cisco devices being used.