Transparent Bridging

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The most common bridging technology is transparent bridging. Other forms of bridging exist, such as source-route bridging, source-route transparent bridging, and source-route translation bridging, but are out of the scope of this book and will not be covered. A transparent bridge learns the MAC address of each host on a segment by looking at the source MAC address in the frame and forwarding the frame out only the port where the destination MAC address resides.

As the transparent bridge discovers the MAC addresses, it builds a table mapping the MAC address to the port on which it was learned. This way, the bridge does not have to rediscover the destination port every time a frame is received. As a frame is received, it can refer to the table and thus know the destination port and forward the frame appropriately. In essence, bridges build their table on source MAC addresses and filter on the destination MAC address. Bridges help segment traffic to local resources and save bandwidth. Each port on a bridge is a collision domain, and all ports on a bridge constitute a broadcast domain (meaning that all ports on a bridge belong to the same IP or IPX network). Figure 5-1 illustrates how ports on a bridge relate to collision domains and broadcast domains.

Figure 5-1. A Standard Bridge

Although both bridges and switches operate with similar technique, switches have more functionality.

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