Bridging Methods

There are two primary methods of bridging:
· Transparent bridging (mainly used with Ethernet LANs), also called spanning tree bridging (STB)
· Source-route bridging (SRB) (used in 802.5 LANs). Then, from these two primary methods of bridging, there are other methods listed as follows:
Source-route transparent bridging (SRT)
Source-route - translational bridging (SR-TB)
Tunnel bridge (IP encapsulation)
All of these bridging methods are supported by the IBM 2210.
In the following topics, we provide a summary description of these bridging
methods.

Transparent Bridging (STB)

A transparent bridge is also called a spanning tree bridge (STB).
Transparent bridging is normally used to connect LAN segments. It is specified in the ISO 8802-1 standard.

This form of bridging could also be used for connection of token-ring LAN segments, although this is not common.

Transparent bridging is based on the principle that a sending device can transmit a frame to a receiving device on a LAN network without having any knowledge of the location of, or the path to, that receiving device.

Transparent bridges within a network are responsible for forwarding the frame to the correct destination, making the determination of whether a frame should be forwarded based on MAC sub-layer destination address.

Transparent bridges achieve this by building and maintaining a filtering database that acts as a forwarding table for received frames. They build their database by copying all frames from the LANs to which they are attached and learning the location of devices by inspecting the MAC sub-layer source address in each received frame.

Figure: 1 Transparent Bridging

Figure 1 illustrates how a transparent bridge will build up its filtering database. When the bridge receives a frame from device D1 on port A, it learns that D1 is reached via the LAN on port A. Similarly, if a frame arrives from device D7 on port B, it learns that D7 is reached via the LAN on port B.

For each new source address the bridge sees on the LAN, it adds an additional entry in its database. In time a full picture is built up of all devices on the two LANs and via which port they are reached.

The bridge uses its filtering database to determine if an incoming frame should be forwarded or discarded. This is done by examining the MAC sub-layer destination address of each frame and comparing it to the list of addresses in the filtering database:
· If the destination address is not in the database, the frame is forwarded on each port except the receiving port.

. If the destination address is in the database and the frame was received on a port associated with the address, the frame is discarded.

· If the destination address is in the database and the frame was received on a port not associated with the address, the frame is forwarded to the associated port for this destination address in the database.

Transparent bridges require that there be only a single active path between any two LANs in an internetwork. This requirement is to ensure that frames do not loop in such a way that they are seen on both ports of a bridge. If this happens, the bridge will be unable to forward the frames correctly to their destination.

Transparent bridges support and use spanning tree protocol, which ensures a loop-free topology between all the transparent bridges within the network.