Bridges

 

Introduction.  While bridges are really seldom used anymore in my experience and understanding, in order to be thorough I will give them a brief mention.  A bridge connects two network segments and also filters the traffic based on the MAC - Media Access Control address on the packet.

The following diagram represents a bridge.  The MAC addresses are represented using hexadecimal numbers and are next to each computer.

 

 

The following bulleted list gives the major steps that a bridge follows when filtering packets.  When a packet is sent on the network and received by the bridge it:
  • Checks the source and destination MAC addresses and determines which segment each is on.
  • If the destination address isn't in the bridge's address table it forwards it to both segments.
  • If the source address isn't in the bridge's table it adds it to the table.
  • If the destination address is in the bridge's table and not in the same segment as the source address then it forwards the packet to the appropriate segment.
  •  If the destination address is in the bridge's table and in the same segment as the source address then it doesn't forward the packet
  • Bridges forward broadcast messages which are addressed to the broadcast address FF-FF-FF-FF-FF-FF.

A bridge is often called a transparent bridge on an Ethernet network because the computers on the network are not aware it is there.

On Token Ring networks, source routing bridges can be used which rely on the host computer to make the routing decision.

Some bridges can connect network segments using different transmission media.  These bridges are called translation bridges.

Bridges operate on layer 2, the data link layer, of the OSI model.  Non-routable protocols such as NetBEUI can cross bridges due to this.  They regenerate messages at the packet level.

Networks can have more than one bridge which improves fault tolerance.  But having multiple bridges can lead to bridging loop difficulties which occur because there may be multiple paths between two points.  The STA - Spanning Tree Algorithm was developed to overcome the bridging loop problem by identifying preferred unique paths for connecting different segments.