3.2 Layer 2 Protocol Overview - LAN Protocols
3.2.7 Troubleshooting Token Ring
Token Ring networks employ several mechanisms for detecting and compensating for network faults. For example, one station in the Token Ring network is selected to be the active monitor. This station, which can potentially be any station on the network, acts as a centralized source of timing information for other ring stations and performs a variety of ring maintenance functions. One of these functions is the removal of continuously circulating frames from the ring. When a sending device fails, its frame may continue to circle the ring. This can prevent other stations from transmitting their own frames and essentially lock up the network. The active monitor can detect such frames, remove them from the ring, and generate a new token.

The star topology of the IBM Token Ring network also contributes to overall network reliability. Because all information in a Token Ring network is seen by active MAUs, these devices can be programmed to check for problems and selectively remove stations from the ring if necessary.

A Token Ring algorithm called beaconing detects and tries to repair certain network faults. Whenever a station detects a serious problem with the network (such as a cable break), it sends a beacon frame. The beacon frame defines a failure domain, which includes the station reporting the failure, its nearest active upstream neighbor (NAUN), and everything in between. Beaconing initiates a process called autoreconfiguration, where nodes within the failure domain automatically perform diagnostics in an attempt to reconfigure the network around the failed areas. Physically, the MAU can accomplish this through electrical reconfiguration.

This table (PDF 15 kB) outlines problems commonly encountered on Token Ring networks and offers general guidelines for solving those problems.

 

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For further information about Token Ring networks, a good reference is:
http://www.optimized.com/
COMPENDI/L1-TRing.htm