This chapter explains some of the basics involved with enabling QoS on a Catalyst switch. QoS does not create additional bandwidth. Prioritization of certain applications and protocols are becoming essential in today's network. Converged networks that include voice, video, and legacy mainframe traffic have specific delay and bandwidth requirements. Hence, the need for QoS is critical.

This chapter primarily focused on QoS implementation on the Catalyst 6500 switch. In a nutshell, incoming traffic with QoS label at Layer 2 or at Layer 3 is received on the ingress port. The ingress port either allows for the incoming traffic to retain its label or assigns one to the traffic. If trust-cos is enabled on the port, the traffic is forwarded to input scheduling. The next step in the process is to forward the traffic to the switching engine (PFC/PFC2). The PFC/PFC2 marks each frame or packet with an internal DSCP value. If PFC is configured for policing, it will ensure the traffic is within the policy guidelines. The traffic is finally forwarded to the egress port, where the internal DSCP value is marked back to the original label on the traffic. The traffic is put in the egress queue for output scheduling, and subsequently is serviced by WRR.