However, most often such incompatible networks are routed in between, not bridged.Ī 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.
between ARCNET with local addressing and Ethernet using IEEE MAC addresses, requiring translation. Non-transparent bridging is required especially when the frame addressing schemes on both sides of a bridge are not compatible with each other, e.g. Now, if A sends a frame addressed to C, the same procedure will be used, but this time the bridge will not create a new forwarding-table entry for A's address/port because it has already done so.īridging is called transparent when the frame format and its addressing aren't changed substantially. Two-way communication is now possible between A and B without any further flooding to the network. Host C or any other hosts on port 3 are not burdened with the response. The bridge already has A's address in its forwarding table so it forwards the response only to port 1. On the return path, the bridge adds an address and port number entry for B to its forwarding table. Host B recognizes a destination address match and generates a response to A. Host C examines the destination address and ignores the frame as it does not match with its address.
The bridge examines the destination address of the frame and does not find it in its forwarding table so it floods (broadcasts) it to all other ports: 2 and 3. The bridge examines the source address of the frame and creates an address and port number entry for host A in its forwarding table. A sends a frame addressed to B to the bridge. A is connected to bridge port 1, B is connected to bridge port 2, C is connected to bridge port 3. As an example, consider a bridge connected to three hosts, A, B, and C. Transparent bridging can also operate over devices with more than two ports. 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. If the bridge determines that the destination host is on another segment on the network, it forwards the frame to that segment. A bridge reads a frame's destination address and decides to either forward or filter. In the context of a two-port bridge, the forwarding information base can be seen as a filtering database.
Digital Equipment Corporation (DEC) originally developed the technology in the 1980s. Both source and destination addresses are used in this process: 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. By means of these flooded frames, a host on the destination network will respond and a forwarding database entry will be created. If a destination address entry is not found in the table, the frame is flooded to all other ports of the bridge, flooding the frame to all segments except the one from which it was received. The table starts empty and entries are added as the bridge receives frames. Transparent bridging uses a table called the forwarding information base to control the forwarding of frames between network segments.