Transparent Bridges

The Concept Of "Transparency"

When a NIC (Network Interface Card) receives a valid packet, the normal behavior is for the NIC to check to see if the Destination DLC Address in the packet is one that it has been configured to recognize. If the destination DLC Address is one that the NIC has been configured to recognize, then the packet is passed up to the next higher layer of processing.

Bridges, on the other hand, listen to the network in promiscuous mode, meaning that they accept all packets, regardless of the packets' addressing. On a side note, this promiscuous mode is the same thing that allows your network analyzer to capture all the packets going across the network. The bridge then looks up each packet's Destination DLC Address in its internal tables to find out which port the Destination NIC is attached to. Finally, it forwards the packet onto only the necessary port. In the case of a broadcast message, the bridge forwards the packet onto every port except the port that the packet came in on. Promiscuous listening is the key to the bridge's transparent operation. Since the bridge effectively "hears" all packets that are transmitted, it can decide whether forwarding is necessary without any special behavior from the individual stations.

Examples Of Transparent Bridging Behavior

Take, for example, a simple network consisting of a four-port transparent bridge with five stations attached to it. The ports on the bridge shall be numbered one through four, with Station A and Station B on port 1, no station on port 2, Station C on port 3, and Station D and Station E on port 4. (see Fig. 1). If you are using a non-graphical browser, it may help to draw this out on paper.


What happens when Station A transmits a packet destined for Station C? The bridge, which is listening in promiscuous mode, examines its internal tables (let's not worry about how the data gets into the internal tables just yet) and determines that Station C is attached to port 3. It then forwards the packet only onto port 3. (see Fig. 2). Consider what will happen if Station D tries to send a packet to Station E while this is going on. With the bridge, there will be no traffic on segment 4, and the conversation will proceed. If there were no bridge, Station D would not be able to send because Station A would have access to the media.


If Station A transmits a packet that is destined for a station that is on the same port as Station A, for example, Station B, the bridge will realize that there is no reason for it to forward the packet onto any of its ports (see Fig. 3).


If a Station, for example, Station B, transmits a broadcast or multicast packet, the bridge will forward the packet onto all ports (see Fig. 4).


Definition Of A "Bridge"

So, to sum up, here is a concise definition of a transparent bridge:

A bridge is a device that operates at the Data Link Layer of the OSI model. It selectively forwards frames based on an examination of the Data Link Layer addresses in the frames. Any device that fits this description is acting as a transparent bridge.

(This means that a Layer 2 Switch is, essentially, performing a bridging function. Switches are discussed in the Switch Technology section)

This definition has an important implication: A bridge doesn't care what upper layer protocols are being used. A bridge will forward packets correctly whether they are NetWare, TCP/IP, AppleTalk, or even DECnet. As long as the bridge has the right kind of NIC (Ethernet, Token Ring, FDDI, etc...) it can forward properly. Because of this quality, bridges are said to be protocol independent.

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