What is an Ethernet Bridge?
By Stephen Bucaro
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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