Description: The link class controls the behavior of a link when a failure is detected on the link or on the node connected to the remote end of the link. The link class decides which values the dtmLinkStateNodeStatus object can take.

A link is monitored with the following mechanisms:

- The underlying trunk interface will detect when the signal on the receiving interface disappears or is impossible to use due to errors. This is called a Signal Failure.

- The communication with the node at the remote end of the link is monitored by periodically sending messages, and expecting responses. If no response is received within a reasonable time, a failure is assumed. This is called a Supervision Failure.

The error detection mechanisms can detect failures in one or both directions of a bi-directional link. If a failure is detected in only one direction of a bi-directional link and there is another fully working link in the opposite direction, the remaining working links will all have status `up'.

The link classes are:

normal(1) If either a Signal Failure or Supervision Failure is detected on the link, the link is considered as `down' and its entry is removed from the dtmLinkStateTable.

persistent(2) If a Signal Failure is detected on the link, the link is considered as `down' and its entry is removed from the dtmLinkStateTable. If a Supervision Failure is detected on the link, the object dtmLinkStateIfNodeStatus for the link is assigned `noControl'.

nailed(3) If a Signal Failure is detected on the link, the object dtmLinkStateIfNodeStatus for the link is assigned `downKeep'. If a Supervision Failure is detected on the link, the object dtmLinkStateIfNodeStatus for the link is assigned `noControl'.