Bridging is a forwarding technique use 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 broadcasting to locate unknown devices. Once a device has been located, its ___location is recorded in some fashion so as to preclude the need for further broadcasting.
The utility of bridging is limited by its dependence on broadcasting, and is thus used only in small networks, or small subsets of large networks.
Currently, two different bridging technologies are in widespread use. Transparent bridging predominates in Ethernet networks; while source routing is used in Token-Ring networks.
Transparent Bridging
Transparent bridging refers to a form of bridging "transparent" to the end systems using it, in the sense that the end systems operate in exactly the same way as in a non-bridged network. It is used primarily in Ethernet networks, where it has been standardized as IEEE 802.1d.
The bridging functions are confined to network bridges which interconnect the network segments. First, the network bridges use the spanning tree algorithm to built a loop-free network topology by selectively disabling network segments. This allows broadcasting to occur simply by copying packets; the tree structure insures that loops will not occur and that broadcast packets will therefore not be copied indefinately. Next, the bridges monitor all packets traveling on the network, noting their source addresses, matching these addresses to the interfaces the packets were received on, and recording this information in a table. Finally, the bridges forward packets according to a simple algorithm - if the destination address does not appear in the table, broadcast the packet throughout the network; otherwise, forward it only on the interface recording in the table.
Note that both source and destination addresses are used in this algorithm. Source addresses are used to record entries in the table, while destination addresses are used to lookup entries.
As a simple example, consider a packet exchange between two hosts (A and B) in a bridged network. A begins by transmitting a packet to B. Since B (the destination) does not appear in any of the bridges's tables, the packet is broadcast throughout the network. Each network bridge sees a copy of the packet, and records its source address (A) along with the interface it was received on. Each bridge now has an entry for A in its table. B, having received a packet from A, now transmits a packet in response. Since the bridges all have an entry for A (the packet's destination), no broadcast is required, and the packet is forwarded along the tree from B to A. All the bridges along this path record the packet's source address - B. Two-way communication is now possible between A and B without any further broadcasting. Note, however, that only the bridges along the direct path between A and B possess table entries for B. If a third host (C), not along the direct path, now transmits a packet for B, it will be broadcast throughout the network.
Source Route Bridging
Source route bridging is used primarily on Token-Ring networks, and is standardized in Section 9 of the IEEE 802.2 standard. The spanning-tree algorithm is not used, the operation of the network bridges is simplier, and much of the bridging functions are performed by the end systems, particularly the sources, giving rise to its name.
A field in the Token-Ring header, the Routing Information Field (RIF) is used to support source-route bridging. Upon sending a packet, a host attaches a RIF to the packet indicating the series of bridges and network segments to be used for deliverying the packet to its destination. The bridges mearly follow the list given in the RIF - if a given bridge is next in the list, it forwards the packet, otherwise it ignores it.
When a host wishes to send a packet to a destination for the first time, it needs to determine an appropriate RIF. A special type of broadcast packet is used, which instructs the network bridges to append their bridge number and network segment number to each packet as it is forwarded. Loops are avoided by requiring each bridge to ignore packets which already contain its bridge number in the RIF field. At the destination, these broadcast packets are modified to be standard unicast packets and returned to the source along the reverse path listed in the RIF. Thus, for each route discovery packet broadcast, the source receives back a set of packets, once for each possible path through the network to the destination. It is then up to the source to choice one of these paths (probably the shortest one) for further communications with the destination.
Source Routing Transparent (SRT) Bridging
Source routing transparent (SRT) bridging is a hybrid of source routing and transparent bridging, standardized in Section 9 of the IEEE 802.2 standard. It allows source routing and transparent bridging to coexist on the same bridged network by using source routing with hosts than support it and transparent bridging otherwise.