The primary role of quality of service (QoS) in the campus distribution switch is to manage packet loss. Therefore, the distribution switch should trust differentiated services code point (DSCP) markings on ingress (as these have been previously set by access-edge switches) and perform both ingress (if required and supported) and egress queuing, as illustrated in Figure 15-1.
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Figure 15-1 Campus Distribution Switch Port QoS Roles
The Cisco Catalyst 4500E Supervisor 7-E is a platform well suited to the role of a campus distribution switch and therefore is featured in this design chapter.
Incidentally, the QoS design requirements of a Catalyst 4500E Supervisor 7-E in the role of a distribution switch are generally equivalent to the requirements of a campus core switch.
Cisco Catalyst 4500 QoS Architecture
From a QoS perspective, the Cisco Catalyst 4500-E Supervisor 7-E is nearly identical to the Supervisor 6-E platform and the Catalyst 4500-X, because all of these platforms are Modular QoS command-line interface (MQC) based. However, earlier Catalyst 4500 platforms (such as the Supervisor II-Plus through Supervisor V-10GE) are Multi-Layer Switch (MLS)-QoS-based platforms and are referred to as Classic Supervisors.
Figure 15-2 illustrates the QoS architecture for this Catalyst 4500E Supervisor 7-E (hereafter referred to simply as the Catalyst 4500) platform.
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Figure 15-2 Cisco Catalyst 4500 QoS Architectural Model
QoS is enabled by default on all MQC-based platforms, which includes the Catalyst 4500. In addition, by default, all ports are set to a trust-DSCP/trust-CoS state.
In the MQC-based Catalyst 4500, QoS policies are applied as follows:
- The incoming packet is classified (based on different packet fields, receive port, or VLAN) to belong to a traffic class.
- Depending on the traffic class and configured polices, the packet is policed, which may result in the packet being dropped or re-marked.
- After the packet has been marked/re-marked, it is looked up for forwarding. This action obtains the transmit port and VLAN to transmit the packet.
- The packet is classified in the output direction based on the transmit port or VLAN/marking.
- Depending on the output policies, the packet is policed, and may be dropped or re-marked.
- The transmit queue for the packet is determined based on the traffic class and the configured egress queuing policies.
- The transmit queue state is dynamically monitored via Dynamic Buffer Limiting (DBL) and drop threshold configuration to determine whether the packet should be dropped or queued for transmission.
- If eligible for transmission, the packet is assigned to a transmit queue.
Based on these QoS operations, the design steps for configuring QoS on the Catalyst 4500 in the role of a distribution switch are discussed next.