Transparent Bridging

Digital Equipment Corporation (DEC) was the first to come to market with a transparent bridge in the early 1980s. IEEE eventually incorporated DEC's work into the 802.1D standard. The term transparent bridge was used because the bridge is completely transparent to the end stations that it is interconnecting. Frames that pass through a transparent bridge are not modified: What comes in on an interface will leave exactly the same way on another interface. Transparent bridges perform three basic functions:

• They make forwarding and filtering decisions based on the destination MAC address in a frame.

• They learn where end stations reside in the network.

• They remove loops.

Remember the three main functions of a transparent bridge in the preceding list.

Throughout the remainder of this chapter, I'll use the term switch, instead of bridge, because most people deploy this kind of Layer 2 device in today's networks.

Forwarding and Filtering

As a frame comes into a port, the switch examines the destination MAC address, performs a lookup in its port address table (also called a content addressable memory [CAM] table) for a matching MAC address, and forwards the frame out the appropriate port. If the switch does not see a match, it floods the frame out all its ports except the incoming port. Local broadcasts and multicasts are treated the same way because bridges do not store these MAC addresses in their CAM tables. Because forwarding decisions are made on MAC addresses, these must be unique in a transparently bridged or switched network.

Learning

Besides examining the destination MAC address to make a forwarding decision, the switch also examines the source MAC address in the frame. After examining the CAM table for a match and not finding one, the switch adds the source MAC address to the CAM table with the address's associated port. If the frame is already in the CAM table, the switch resets its aging counter. If a certain MAC address is not seen after a period of time, the entry eventually will be removed from the CAM table.

The advantage of a switch that dynamically learns the addresses of end stations is that you can plug the switch into the network and it will acquire knowledge of the network without human intervention. If you move an end station to a different segment, the switch will realize this and update its CAM table appropriately. When transparent bridges were brought to market, they did not have a learning capability, meaning that you had to manually configure the address table. Of course, today's bridges and switches have the capability to perform their learning function automatically.

Loops

The downside of transparent switches, however, is that no redundant connections or parallel paths are allowed, as shown in Figure 4.1.

Figure 4.1. Transparent switches and Layer 2 loops.

A special protocol developed by DEC, called the Spanning Tree Protocol, is an algorithm that runs on all switches that will, in software, disable any loops found in a transparently switched network. On completion, only one active path will exist between one switch and any other switch in the network.