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Definition - What does Bridging mean?

Bridging is a technique used for message forwarding in packet switched networks. In contrast to routing, bridging uses the destination address placed inside the message header to locate unknown network devices.

The bridging technique does not make any assumptions to find the network devices over local area networks (LAN) or Ethernet. Instead, bridging uses flooding, meaning that it is only designed for use on LANs. Once a network device is located, its address is automatically stored in table for future purposes.

Bridging only controls and manages the network traffic within a single network segment rather than trying to manage adjacent segments.

Techopedia explains Bridging

Bridging uses a forwarding database for recording and storing network device addresses. This forwarding database also contains frames that are used to fetch the network devices addresses if the forwarding database is empty. The frame is forwarded to all other network devices with the exception of the source device. When the proper device is found, a destination entry will be created in forwarding database.

Bridging is best understood using an example:

  • There are three hosts (X, Y and Z) and a bridge.
  • The bridge has three ports (1, 2 and 3).
  • Host X is connected to port 1, Host Y is connected to port 2 and Host Z is connected to port 3 of the bridge.

In this situation, suppose Host X sends a frame for Host Y to the bridge. Assuming that this is a newly created bridge, a sequence of events is set in motion:

  1. Before performing any action, the bridge will examine the incoming frame and will create an entry for Host X in its forwarding table against port 1.
  2. The bridge will search its forwarding table. (Let’s assume it doesn’t find an address for Host Y.)
  3. The bridge floods the frame to all its ports to locate the addressed device. Host X is left out because it originated the frame.
  4. The frame will be completely ignored by Host Z, but Host Y will accept it and provide its own port number and address.
  5. Upon receiving Host Y’s port number and address, the bridge makes an entry in forwarding table. The information in will also be sent to Host X.

Thus, a two-way communication path has been established between Host X and Host Y, and this path will be readily available in the future.

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