From OpenNMS

Link monitoring is made up of three parts, Discovering, Polling and Correlation.

Discovery

First at Provision time, an EndPoint detector will detect whether a given node is eligible for EndPoint monitoring. This is based on the sysOID in the EndPoint configuration. If a node is detected as an EndPoint it will be assigned as en EndPoint service.

Polling

Once a node has the EndPoint service it will be monitored like any other service. It will determine the EndPoint availability, based on the EndPoint configuration.

Correlation

Link Provisioning Adapter will also listen for these events:

• Node Down
• Interface Down
• Node Lost service
• Node Gained service
• Node Regained service
• Service Managed
• Service Unmanaged
• Service Deleted
• Suspend Polling Service
• Resume Polling Service

On any of these events it will correlate these events and evaluate the status of the effected links.

After the link status has been checked, if the link status has changed update it's status. It will update the link status by sending out a 'dataLinkFailed' or 'dataLinkRestored' event, see Link_Status_Updating_Specification.