Segmenting with Bridges

A bridge is a hardware segmentation device that operates at the first two layers of the OSI reference model---the physical and data link layers. Bridges segment a LAN's media access domain. Therefore, installing a bridge between two LAN hubs results in two media access domains that share a common MAC broadcast domain.

In general, all bridges work by building address tables. These tables are built and maintained by a bridge. Each is populated with a two-dimensional array or table. The bridging table maintains an up-to-date listing of every MAC address on the LAN, as well as the physical bridge port connected to the segment containing that address.

In operation, the bridge listens to all LAN traffic. The source and destination MAC addresses of each frame received by the bridge are examined. This allows the bridge to learn which MAC addresses reside on which port and, consequently, which LAN segment.

The destination address is hashed against the bridging table to identify the appropriate port to transmit it from. If the MAC address exists on the same LAN segment that the frame came from, the bridge needs to do nothing with it; it safely assumes that the frame has already been carried to its intended destination.

If the bridging table identifies that MAC address as being on a different segment, however, the bridge then forwards that frame to that segment. It is important to note that the bridge, as far as media access is concerned, must adhere to the media access protocol. In a token-passing network, the bridge must await the token before it can forward the frame. In a contention-based LAN, the bridge must compete for available bandwidth before it can forward the frame.

It is quite possible that the bridge will occasionally receive a frame addressed to a MAC address that the bridge doesn't know about. This can happen when a new device is connected to the network, a bridge's bridging table is "lost," or a new bridge is installed. In such cases, the bridge will propagate that frame to all its attached LAN segments, except for the one the frame came from.

Bridging, in an IEEE 802-compliant LAN, occurs at the MAC layer. For this reason, bridges are frequently referred to as MAC bridges. MAC bridging is an unnecessarily broad technical term. It effectively describes the layer at which the device operates but does not describe its functionality. In fact, there are three types of MAC bridges:

Routers and LANs

• Transparent bridges

• Translating bridges

• Speed-buffering bridges

Transparent Bridges

Transparent bridges link together segments of the same type of LAN. The simplest transparent bridge contains just two ports, but transparent bridges may also contain more ports. Figure 3-7 illustrates how a transparent bridge isolates the traffic of two LAN segments by creating two media access domains.

Figure 3-7: Transparent bridges segment the media access domain of a single LAN architecture.

Figure 3-7: Transparent bridges segment the media access domain of a single LAN architecture.

The transparent bridge segments one LAN with one communications channel into two distinct communications channels within a common architecture. This is significant because it means that a bridge can reduce the number of devices in a media access domain by creating two such domains.

It is important to note that transparent bridges do not segment a LAN's MAC broadcast domain. Therefore, in Figure 3-7, MAC broadcasts are still carried throughout the entire LAN. Despite this, the LANs on each side of the bridge function as separate media access domains.

Translating Bridges

A translating bridge, sometimes also referred to as a translational bridge, works in exactly the same manner as a transparent bridge, but it has the added capability to provide the conversion processes needed between two or more LAN architectures. It does this by literally translating the frames of one LAN architecture into the frame structure of another. This is useful for interconnecting Token Ring and Ethernet devices.

Figure 3-8 illustrates using a translating bridge to interconnect a Token Ring and Ethernet LANs. The stations on both LANs may communicate with each other through the bridge as easily as they communicate among themselves.

Note In Figure 3-8, the Token Ring LAN is depicted as a ring, and the Ethernet is depicted as a bus. This visually reinforces the differences between these two LAN architectures that would not otherwise be evident if they were illustrated using the more familiar star topology.

Figure 3-8: Translating bridges interconnect dissimilar LAN architectures.

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Figure 3-8: Translating bridges interconnect dissimilar LAN architectures.

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The Token Ring and Ethernet LANs in Figure 3-8 retain separate media access domains. Given the radical differences in their media access arbitration techniques, this shouldn't be surprising. What may be surprising, however, is that the bridge unifies their MAC broadcast domains! Therefore, a Token Ring-connected computer can send MAC broadcasts to Ethernet-connected machines.

Perhaps a more useful application of translation bridging is using a more robust LAN architecture as a backbone for client/server LANs. It is quite common, for example, to use FDDI to interconnect Ethernet segments. This is illustrated in Figure 3-9.

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