Local Networks and the Internet: From Protocols to Interconnection

Chapter 13

Virtual Local Networks

13.1. Definition

The IEEE level 2 local networks, as have been shown in the previous chapters, can work without any particular configuration.

— It is not necessary to give the device a level 2 address: during its construction, it receives a unique MAC address (see section 2.3, page 27). On the other hand, this address is only used to assign this equipment, preventing any ambiguity. It does not locate it in the network. The natural distribution properties on the local networks will do this.

— In the case of more complex networks, the interconnection equipment (bridges, switches, etc.) always give the broadcast property needed to locate the equipment. For instance, they automatically build the list of stations available in each bridge to reduce the point-to-point traffic (see section 5.2, page 116).

— The Spanning Tree algorithm used in almost all interconnection equipments at level 2 builds a topology without loops and resistant to line failures. This algorithm shows the philosophy deployed by the IEEE. It is not guaranteed that the network will function optimally without any configuration parameters linked to the Spanning Tree in the bridges. The default values will hinder the functioning of the network, but the network still works nevertheless.

The use of semi-bridges linked through specialized links extends the network coverage in a country. The level 2 networks will have problems related to scale, however, when the amount of equipment and the ...