Understanding QoS-based Packet Processing

A switch’s packet processing logic is divided into three main stages:

• The ingress logic (corresponding to the port-related ingress functions)
• The centralized logic (corresponding to switch-wide processing and forwarding functions)
• The egress logic (corresponding to the port-related egress functions)

QoS is implemented in all of the above processing stages:

• In the logic associated to the ingress port, the input traffic’s classification can be trusted, or it can be remarked per the network administrator’s configuration.
• In the centralized logic, traffic can be filtered and QoS policies can be applied to it: traffic can be remarked and/or policed.
• In the logic associated to the egress port, traffic is scheduled for transmission in an egress queue, in which it can be shaped per the network administrator's configuration.

Figure 20-3 below shows the logical sequence of the QoS operations applied by the forwarding hardware to a packet:

Figure 13-3: QoS Packet Flow