Bridging (networking)

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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 broadcasting to locate unknown devices. Once a device has been located, its location is recorded in a routing table where the MAC address is stored alongside its IP Address so as to preclude the need for further broadcasting. This informations are stored in ARP table

The utility of bridging is limited by its dependence on broadcasting, and is thus only used in local area 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. Thus, bridging allows you to connect two different networks seamlessly on the data link layer, e.g. a wireless access point with a wired network switch by using MAC addresses as an addressing system. A bridge and switch are very much alike.

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 as if the bridge isn't there in the way that matters: bridges segment broadcasts between networks, and only allows specific addresses to pass through the bridge to the other 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. The active parts of the network must form a tree. This can be achieved either by physically building the network as a tree or by using bridges that use the spanning tree protocol to build a loop-free network topology by selectively disabling network broadcast addresses. If one computer on network A sent a broadcast packet (packet with destination mac address FF:FF:FF:FF:FF:FF) to address FF:FF:FF:FF:FF:FF, the bridge would stop this from getting to network B. Note we have 3 addresses: source address and target address of the packet and the address where we send the packet. The mac address FF:FF:FF:FF:FF:FF is the broadcast address for both networks; when a frame is sent to this address, the frame is then resent out on every available port on that specific network segment. This method allows the bridge to only switch frames that have a specific MAC address, that is, one that is not mac FF:FF:FF:FF:FF:FF. When an address is specified and a frame is sent, the bridge automatically switches the frame to both network segments while noting the source MAC addresses' home segment. This allows the bridge to send frames across the networks, by recording and resolving MAC addresses of devices on each side. Next, the bridges monitor all frames traveling on the network, noting the frame's source addresses in a table, and then broadcasting the frame with a specific destination (not broadcast) address to the other networks, effectively rebroadcasting it to every device available on every network segment until the specified destination is found. Without broadcast segmentation, the bridge would get caught in an infinite loop.

Note that both source and destination addresses are used in this algorithm. Source addresses are recorded in entries in the table, while destination addresses are looked up in the table and matched to the proper segment to send the frame to.

As an example, consider two hosts (A and B) and a bridge (C). The bridge has two interfaces, (C1, C2). A is connected to the C1 and B is connected to the C2. Note the physical connection is A - C - B, since C has two ports. A sends a frame to (C), and C records the source MAC address into its table. The bridge now has an address for A in its table, so it forwards it to B by broadcasting it to FF:FF:FF:FF:FF:FF, or every address possible. B, having received a packet from A, now transmits a packet in response. This time, the bridge has A's address in the table, so it records B's address sends it to A's unique MAC address specifically. Two-way communication is now possible between A and B without any further broadcasting. Note, however, that only the bridge along the direct path between A and B possess table entries for B. If a third host (D), on the same side as A sends a frame to B, the bridge simply records the address source, and broadcasts it to B's segment.

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 protocol is not used, the operation of the network bridges is simpler, 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 delivering the packet to its destination. The bridges merely 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, one for each possible path through the network to the destination. It is then up to the source to choose one of these paths (probably the shortest one) for further communications with the destination.