The IEEE 802.1D specification includes the Spanning Tree Protocol (STP), a mechanism that ensures that a forwarding loop never occurs. The STP uses Bridge Protocol Data Units (BPDUs) to automatically configure individual ports on bridges in either a forwarding or blocking state. BPDUs are messages sent by the bridges to a reserved multi-cast MAC address (01-80-C2-00-00-00 for Ethernet) on which all transparent bridges are listening. In the blocking state, the port does not learn or forward received frames. The end result of the STP is a loop-free bridged environment that exists at all times, regardless of LAN segment topology changes.
Bridges and switches are data communications devices that operate principally at Layer 2 of the OSI reference model. As such, they are widely referred to as data link layer devices.
Bridges became commercially available in the early 1980s. At the time of their introduction, bridges connected and enabled packet forwarding between homogeneous networks. More recently, bridging between different networks has also been defined and standardized.
Several kinds of bridging have proven important as internetworking devices. Transparent bridging is found primarily in Ethernet environments, while source-route bridging occurs primarily in Token Ring environments. Translational bridging provides translation between the formats and transit principles of different media types (usually Ethernet and Token Ring). Finally, source-route transparent bridging combines the algorithms of transparent bridging and source-route bridging to enable communication in mixed Ethernet/Token Ring environments.
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