However, bridges may be used to connect multiple LANs together into complex internetwork topologies and MAC addresses have no hierarchical structure that would assist with routing. Transparent bridges attempt to maintain forwarding databases called filter tables, which indicate to which port a frame with a given destination MAC address should be directed or indeed whether it should be discarded. Because LAN addresses are dynamic, in the sense that the point of attachment of a machine may be changed anytime, transparent bridges try to learn where particular addresses are currently located by observing the source addresses of frames passing through. A frame with source address x arriving on port k of a given bridge, will cause that bridge to create or update an entry in its forwarding database, suggesting that any frame addressed to destination x should be sent on through port k. This is called backward learning. A bridge will flood frames whose destination addresses are unknown to all ports other than the one on which the frame arrived. However, as the filter table is constructed, the bridge becomes more discriminating, and sends each frame only to the network on which its destination is known to reside. Unfortunately, when two LANs are connected by more than one bridge looping behaviour can set in if this simple procedure is not controlled.
Once the internetwork becomes sufficiently complex, a bridge will often see frames that are not destined for any LAN to which it, itself, is connected. The bridge must be able to identify the LAN on which the next bridge to which the frame should be sent resides. 802.1(D) defines a spanning tree protocol (STP) and spanning tree algorithm for transparent bridges in an internetwork allowing them to construct a spanning treewhich they all share, in order to prevent the looping behaviour mentioned above. The bridges use this tree to determine the route that any frame crossing the bridged internetwork must follow. The bridges send BPDUs (bridge PDUs) to each other to determine, firstly a root bridge, and secondly, a preferred path from each other bridge through a specified root port back to the root bridge. If multiple bridges are interfaced to a given LAN, only one, the designated bridge is allowed to pass frames moving away from the root into that LAN, preventing looping. Even once the spanning tree has been established, 802.1(D) bridges continue to pass BPDUs to each other to keep the tree updated. BBPDUs have an 802.3 format with an LLC payload, directed to the reserved multicast address 01-80-C2-00-00-00 .
Figure 1 Spanning tree bridging of LAN internetwork
Consider the internetwork shown in Figure 1a. Notice that there are a number of places where bridges using backward learning could generate loops. Using the bridge protocol, the bridge with the lowest serial number becomes the root bridge. Other bridges use the rated capacity of each port as a cost measure for crossing the LAN to which the port is attached, and, based on this measure, calculate a spanning tree such as that shown in Figure 1b. In this tree, each LAN gets one designated bridge, with a designated port, which has the sole right to forward frames coming from the root direction to that LAN. Some bridges, such as Bridge 3 in this example, may be entirely removed from the tree and are no longer used for forwarding frames, although they may still prove useful if the tree has to be recalculated, say, due to a failure. A bridge that is part of the tree has a single root port through which all frames passing to or from that bridge to the root bridge must pass.
One significant problem with spanning tree protocols is the time it takes to create a stable topology if some event occurs which requires the tree to be recomputed (time can be a sizable fraction of a minute).