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

12.4. IEEE 802.1Q Switch Operation

We can understand all of the application environments, define all of the VLAN association rules, and specify frame and tag formats all we want, but ultimately a real-life VLAN-aware switch has to implement all this stuff. Figure 12-10 depicts a model of the operational flow of frames through a VLAN-aware switch.

Figure 12.10. IEEE 802.1Q switch operation flow

A switch comprises some number of physical port interfaces that can be used both to receive and transmit frames. The fundamental job of a VLAN-aware switch is to receive frames and decide, on a frame-by-frame basis:

• Onto which port(s), if any, to forward the frame

• Whether forwarded frames should be transmitted in tagged or untagged format

To achieve these ends in a VLAN context, the switch must maintain state information for each VLAN that indicates which ports attach to LANs that are needed to reach members of that VLAN. This set of ports, known as the member set, is determined by the VLAN association rules in effect. Ports in the member set for a given VLAN can be expected to receive and transmit frames belonging to that VLAN; ports not in the member set should generally not be receiving and/or transmitting frames for that VLAN. The member set for a VLAN may be configured by a human administrator or, more likely, learned dynamically through an automatic mechanism such as the GARP VLAN ...