Thrashing Links

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A discussion of fault-tolerant networks would not be complete without mentioning thrashing links. Thrashing is a phenomenon that occurs when paths are quickly switched back and forth. For example, in a network with two paths (primary and backup), the primary path fails and the backup path is placed in service. The primary path self-heals and is switched back into service, only to fail again.

Thrashing is primarily caused by intermittent failures of primary paths and preprogrammed switchback timers. In order to overcome thrashing, the acting protocols and switches must use hold-down times. For example, some programs allow 1 minute for the first hold-down time and set a trigger so that, on the second switchback, operator intervention is required to perform a switchover and prevent thrashing.

Checkpoint 

Answer the following questions.

1. Why is it that MPLS is in need of additional degrees of protection other than the protections provided by other protocols?

2. Name two types of RSVP-TE network protection.

3. Name the primary method used to avoid thrashing links.

Answers: 1. Because there is no protocol responsible for ensuring the quality of the link tunnel or call placed on an MPLS link; 2. link protection and node protection; 3. hold-down time: the process by which a secondary path is kept in place for a minimum amount of time prior to switching back to the primary path.

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