The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

13.5. Priority Operation in Switches

In this section, you consider how a switch deals with frame priority.

If you don't invoke any priority mechanisms, the operation of a switch is quite straightforward. When frames arrive on a port, the switch makes its filtering/forwarding decision and determines onto which port(s) the frame should be forwarded. A single-queue model can be used for both input and output ports, as depicted in Figure 13-8. Frames are operated on and forwarded in a first-come first-served manner.

In this scenario, the switch does not attempt to determine priority and handles all frames equally.

Figure 13.8. Single-queue model

13.5.1. The Ordering Invariant–Redux

Switches, being Data Link layer internetworking devices, must maintain the ordering invariant. Before we considered priority, this invariant was posited as a constraint on the order of frames between any Source-Destination Address pair. However, the actual invariant is that ordering must be maintained on the flow between any Source-Destination Address pair for frames of a given user priority [ISO95]. If there is no concept of user priority (e.g., in an old-fashioned Ethernet environment), this invariant requires you to maintain strict order for all frames sent between each address pair. However, if you allow frames to have an associated user priority, then you need only to maintain the order for those ...