The Spanning-Tree Protocol is maintained in each Layer 2 device, a switch or a bridge, allowing the device to ensure that it has only one path back to the root bridge. Any redundant paths will be blocked, as long as the Layer 2 device continues to see the primary path. The health of this primary path is ensured by the receipt of spanning-tree updates (called bridge protocol data units, or BPDUs). As soon as the Layer 2 device fails to see the updates, panic sets in and the device removes the block on the redundant path, falsely believing it to be the only path available. The block on the redundant path is removed after several updates have been missed, after the MaxAge timer has been exceeded. This ensures some stability in the network. However, if this problem occurs, in a short time, spanning-tree loops and broadcast storms will cause the network to seize up and die.
This is one of the reasons that the industry moved toward a routed network solution. However, organizations are beginning to reintroduce Spanning-Tree Protocol into their switched environments without any problems. This is a result of the vast improvements in the hardware, the increased bandwidth, and the use of VLANs to reduce the size of the broadcast domain, and, in a Cisco implementation, the spanning-tree domain. Another factor that positively affects the stability of large flat networks is the increasing use of multicast addresses. Multicast addresses are read only by devices belonging to the multicast address and are forwarded only to segments that contain such devices. Refer to the Cisco Design Guides, on the Cisco Connection Online (CCO) web page, for more information on these topics.
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