3.2 Layer 2 Protocol Overview - LAN Protocols
3.2.10 FDDI troubleshooting
FDDI supports a self-healing capability in its dual-attached configuration by using the two rings to wrap around a failure. When this capability is used, the actual direction of the flow of the token through the rings will vary from the norm. Given that wraparound is indicative of an error state, this shouldn't be surprising. To better understand the FDDI self-healing capability, consider the example presented in Figure . This figure demonstrates what would happen if a cable failed. The network components detect a break in the ring, and automatically seek to restore the integrity of the ring by wrapping around the failed component. In this manner, service is restored to as much of the original network as possible.

It is important to note that FDDI is equally capable of recovering from failures of physical devices on the network, as well as just cable breaks.

Figure outlines problems commonly encountered on FDDI networks and offers general guidelines for solving those problems.

 

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For further information about FDDI networks, a good site is:
http://www.optimized.com/
COMPENDI/L1-FDDI.htm