Bridging is a forwarding technique used in packet-switched computer networks. Unlike routing, bridging makes no assumptions about where in a network a particular address is located. Instead, it depends on flooding and examination of source addresses in received packet headers to locate unknown devices. Once a device has been located, its location is recorded in a table where the source address is stored so as to avoid the need for further flooding. The utility of bridging is limited by its dependence on flooding, and is thus only used in local area networks.
Bridging generally refers to transparent bridging or learning bridge operation which predominates in Ethernet. Another form of bridging, source route bridging, was developed for token ring networks.
A network bridge connects multiple network segments at the data link layer (Layer 2) of the OSI model. In Ethernet networks, the term bridge formally means a device that behaves according to the IEEE 802.1D standard. A bridge and a switch are very much alike; a switch being a bridge with numerous ports. Switch or Layer 2 switch is often used interchangeably with bridge.
Advantages of network bridges
- Simple bridges are inexpensive
- Isolate collision domains with microsegmentation
- Access control and network management capabilities
- Bandwidth scales as network grows
Disadvantages of network bridges
- Does not limit the scope of broadcasts
- Does not scale to extremely large networks
- Buffering and processing introduces delays
- A complex network topology can pose a problem for transparent bridges. For example, multiple paths between transparent bridges and LANs can result in bridge loops. The spanning tree protocol helps to reduce problems with complex topologies.