Bridging (networking)

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A network bridge is a computer networking device that creates a single aggregate network from multiple communication networks or network segments. This function is called network bridging.[1] Bridging is distinct from routing. Routing allows multiple networks to communicate independently and yet remain separate, whereas bridging connects two separate networks as if they were a single network.[2] In the OSI model, bridging is performed in the data link layer (layer 2).[3] If one or more segments of the bridged network are wireless, the device is known as a wireless bridge and the function as wireless bridging.


Transparent bridging[edit]


In the context of a two-port bridge, one can think of the forwarding information base as a filtering database. A bridge reads a frame's destination address and decides to either forward or filter. If the bridge determines that the destination host is on another segment on the network, it forwards the frame to that segment. If the destination address belongs to the same segment as the source address, the bridge filters the frame, preventing it from reaching the other network where it is not needed.


Simple bridging[edit]

A simple bridge connects two network segments, typically by operating transparently and deciding on a frame-by-frame basis whether or not to forward from one network to the other. A store and forward technique is typically used so, during forwarding, the frame integrity is verified on the source network and CSMA/CD delays are accommodated on the destination network. In contrast to repeaters which simply extend the maximum span of a segment, bridges only forward frames that are required to cross the bridge. Additionally, bridges reduce collisions by partitioning the collision domain.

Multiport bridging[edit]

A multiport bridge connects multiple networks and operates transparently to decide on a frame-by-frame basis whether and where to forward traffic. Like the simple bridge, a multiport bridge typically uses store and forward operation. The multiport bridge function serves as the basis for network switches.


The forwarding information base stored in content-addressable memory (CAM) is initially empty. For each received ethernet frame the switch learns from the frames source MAC address and adds this together with the ingress interface to build the forwarding information base. The switch then forwards the frame to the interface found in the CAM based on the frames destination MAC address. If the destination address is unknown the switch sends the frame out on all interfaces (except ingress interface). This behaviour is called unicast flooding.



  4. Adaptive switching: a method of automatically selecting between the other three modes.[7][8]

See also[edit]



Further reading[edit]