What is an Ethernet Bridge?

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Bridges are used to divide larger networks into smaller sections. They sit between two physical network segments and manage the flow of data between them. bridges work at layer 2, the data link layer, of the OSI model. A bridge reads a frame's MAC address, and decides to either forward or filter the frame from crossing the bridge to the other part of the network.

Learning Bridge

The MAC addresses of devices on each side of the network can be manually entered into a bridge's forwarding database (or filtering database), however most newer bridges can build their own forwarding database by watching traffic on the network. This is called a learning bridge.

Bridging Loop

One problem with learning bridges is that when more than one bridge is implemented on a network, the bridges can confuse each other. Having more than one bridge in a network can provide fault tolerance. At the same time it provides multiple paths from a source to a destination which can cause frames to continuously loop around the network.

To prevent bridging loops Spanning Tree Protocol (STP) is used. Spanning Tree Protocol uses Bridge Protocol Data Units (BPDU) to communicate information about ports between bridges (and switches) and if it detects a loop it can shut down a port to stop the loop. Each time there is a change in a networks topology, or if a port or bridge fails, STP must recalculate the tree.

Types of Bridges

Most bridges are called transparent bridges because devices on the network need not be aware the bridges are there. The bridges simply provide their forwarding and filtering function transparently. A translational bridge is used to convert the data format for one network to that of another network, for example from Ethernet to token ring and from token ring to Ethernet. A source route bridge is a type of bridge used in token ring networks where the entire path that the data frame is to take through the network is provided in the data frame.

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