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QoS in the Multilayer Switched Network

The QoS implementation for a campus network differs at the Campus Backbone, Building Access, and Building Distribution submodules. Because applying QoS classification at the edge is ideal, Layer 3 edge devices generally perform the following QoS functions:

  • Classification on a per-packet basis

  • Marking

  • Congestion management

  • Congestion avoidance

In general, backbone Layer 3 devices perform the following QoS functions, because backbone devices receive packets after classification and marking:

  • Congestion management

  • Congestion avoidance

For Edge submodules that connect Campus Backbones across MANs or WANs, deploy the following QoS features:

  • Classification on a per-packet basis

  • Policing and/or shaping

  • Fragmentation

  • Compression

  • Congestion management

  • Congestion avoidance

Figure 10-17 depicts the recommended QoS feature deployments.

Figure 17Figure 10-17 Recommended QoS Features in the Enterprise Composite Network Model

Networks with special applications of QoS may not exactly follow these guidelines, but these guidelines are a starting point for any QoS network policy.

QoS in the Building Access Submodule

The Building Access submodule is typically where the trust boundary is formed. In this submodule, the Catalyst switches set or trust the DSCP value of ingress packets for use through the entire network. Catalyst switches may set or trust the DSCP value of ingress packets by comparing ingress packets against an ACL or policer. When using ACLs, the Catalyst classifies and/or marks only packets that match specific criteria, such as TCP port number or IP addresses. In addition, using policers to set or trust DSCP values on ingress packets allows the Catalyst switches to determine the trust behavior by the traffic rate. Traffic that exceeds a specified rate and receives a lower DSCP value than traffic that complies with the specified rate is an example of the use of a policer to mark down DSCP values. These features are useful in differentiating traffic flows instead of differentiating traffic by ingress port.

Furthermore, policing is optional in the Building Access submodule. Catalyst switches in the Building Access submodule layer configured for policing applies traffic conditioning and may optionally classify or mark packets before reaching the Campus Backbone or Building Distribution submodule.

Congestion management is a requirement of the Building Access submodule for all interfaces. Classification only determines the internal DSCP of a packet as it traverses the Catalyst switch. Congestion management on the Catalyst switch acts on the internal DSCP values of packets to schedule and apply congestion avoidance accordingly. Congestion avoidance is another useful feature used in the access layer in preventing low-priority traffic from starving higher-priority traffic out of queues.

Not all the Cisco Catalyst switches support all QoS features. In addition, low-end Catalyst switches support the features with significant restrictions, such as restrictions on ACL size and types of ACLs for classification. Consult the product release notes and documentation before deciding which Catalyst switches to use in the QoS design.

QoS in the Building Distribution Submodule

Classification and marking other than trusting in the Building Distribution submodule is necessary only if the Building Access layer does not classify packets or if the Catalyst switches used in the Building Access submodule do not support adequate features necessary to apply QoS effectively. When applying QoS in the Building Distribution submodule, simply use the same principles used when applying QoS in the Building Access submodule. In this layer, configure all inter-switch links for trusting. In this manner, the Building Distribution submodule switches trust the classification from all other switches.

Policing is optional in the Building Distribution submodule as with any submodule. Policing is useful in constraining traffic flows and marking frames above specific rates. Policing is primarily useful for data flows and voice or video flows because voice and video usually maintains a steady rate of traffic.

Congestion management is necessary on all inter-switch links and any hosts or servers that may connect to the Building Distribution submodule. Congestion management applies proper scheduling of frames for differential service. Congestion avoidance is optional in any submodule but is not a requirement of any submodule.

QoS in the Campus Backbone

The Campus Backbone QoS application is similar to the Building Distribution submodule; use classification and marking in situations where other submodules have not classified or marked traffic. Ideally, there should not be a need to classify or mark traffic in the Campus Backbone submodule. As with other submodules, policing is optional. However, congestion management is a requirement to differentiate traffic flows through the core of the network. Congestion avoidance is optional but recommended to handle congestion effectively.

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