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Since bridging takes place at the data link layer of the OSI Model , a bridge processes the information from each frame of data it receives. In an Ethernet frame, this provides the MAC Address of the frame's source and destination. Bridges use two methods to resolve the network segment that a MAC address belongs to.

  • Transparent bridging – This method uses a forwarding database to send frames across network segments. The forwarding database is initially empty and entries in the database are built as the bridge receives frames. If an address entry is not found in the forwarding database, the frame is rebroadcast to all ports of the bridge, forwarding the frame to all segments except the source address. By means of these broadcast frames, the destination network will respond and a route will be created. Along with recording the network segment to which a particular frame is to be sent, bridges may also record a bandwidth metric to avoid looping when multiple paths are available. Devices that have this transparent bridging functionality are also known as ''adaptive bridges''.


  • Source route bridging – With source route bridging two frame types are used in order to find the route to the destination network segment. Single-Route (SR) frames comprise most of the network traffic and have set destinations, while All-Route(AR) frames are used to find routes. Bridges send AR frames by broadcasting on all network branches; each step of the followed route is registered by the bridge performing it. Each frame has a maximum hop count, which is determined to be greater than the Diameter of the network graph, and is decremented by each bridge. Frames are dropped when this hop count reaches zero, to avoid indefinite looping of AR frames. The first AR frame which reaches its destination is considered to have followed the best route, and the route can be used for subsequent SR frames; the other AR frames are discarded. This method of locating a destination network can allow for indirect Load Balancing among multiple bridges connecting two networks. The more a bridge is loaded, the less likely it is to take part in the route finding process for a new destination as it will be slow to forward packets. A new AR packet will find a different route over a less busy path if one exists. This method is very different from transparent bridge usage, where redundant bridges will be inactivated; however, more overhead is introduced to find routes, and space is wasted to store them in frames. A switch with a faster backplane can be just as good for performance, if not for fault tolerance.



ADVANTAGES OF NETWORK BRIDGES

  • Self configuring

  • Primitive bridges are often inexpensive

  • Reduce size of collision domain

  • Transparent to protocols above the MAC layer

  • Allows the introduction of management - performance information and access control

  • LANs interconnected are separate and physical constraints such as number of stations, repeaters and segment length don't apply



DISADVANTAGES OF NETWORK BRIDGES

  • Does not limit the scope of broadcasts

  • Does not scale to extremely large networks

  • Buffering introduces store and forward delays - on average traffic destined for bridge will be related to the number of stations on the rest of the LAN

  • Bridging of different MAC protocols introduces errors

  • Because bridges do more than repeaters by viewing MAC addresses, the extra processing makes them slower than repeaters

  • Bridges are more expensive than repeaters



BRIDGING VERSUS ROUTING

(Network Layer). This difference means that a bridge directs frames according to hardware assigned MAC Address es while a router makes its decisions according to arbitrarily assigned IP Addresses . As a result of this, bridges are not concerned with and are unable to distinguish Networks while Routers can.

When designing a network, you can choose to put multiple segments into one bridged network or to divide it into different networks interconnected by routers. If a host is physically moved from one network area to another in a routed network, it has to get a new IP address; if this system is moved within a bridged network, it doesn't have to reconfigure anything.


SPECIFIC USES OF THE TERM "BRIDGE"

A description of the Network Bridge in Windows XP is given
here - it allows a Windows XP system to function as a bridge between the various networking devices connecting to it.

Documentation on Linux bridging can be found in the
Linux networking wiki in
here .
Linux bridging allows filtering and routing.


SEE ALSO



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