Transparent bridges connect two or more Ethernet networks. By separating the network into multiple Ethernets, or multiple LAN segments, transparent bridges overcome some of the performance issues covered in the first section of this chapter.
Transparent bridging is called "transparent" because the endpoint devices do not need to know that the bridge(s) exist(s). In other words, the computers attached to the LAN do not behave any differently in the presence or absence of transparent bridges. Before diving into bridging and switching logic, a quick review of a couple of terms about MAC addresses is helpful. The following list defines three terms covered earlier in Chapter 3. These different types of MAC addresses can be treated differently by a bridge or switch.
The IEEE defines three general categories of MAC addresses on Ethernet:
■ Unicast addresses—A MAC address that identifies a single LAN interface card. Today most cards use the MAC address that is burned in on the card.
■ Broadcast addresses—The most often used of IEEE group MAC address, the broadcast address, has a value of FFFF.FFFF.FFFF (hexadecimal notation). The broadcast address implies that all devices on the LAN should process the frame.
■ Multicast addresses—Multicast addresses are used to allow a subset of devices on a LAN to communicate. Some applications need to communicate with multiple other devices. By sending one frame, all the devices that care about receiving the data sent by that application can process the data, and the rest can ignore it. The IP protocol supports multicasting, and when IP multicasts over an Ethernet, the multicast MAC addresses used by IP follow this format: 0100.5exx.xxxx, where any value can be used in the last half of the addresses.
Transparent bridges forward frames when necessary and do not forward when there is no need to do so, thus reducing overhead. To accomplish this, transparent bridges perform three actions:
1. Learning MAC addresses by examining the source MAC address of each frame received by the bridge
2. Deciding when to forward a frame or when to filter (not forward) a frame, based on the destination MAC address
3. Creating a loop-free environment with other bridges by using the Spanning Tree Protocol
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