Prerequisites are resources whose current state must be in the table-relative UP state for another resource to start. Prerequisite checking occurs when the desired or current state of a resource changes, and the resulting desired state is UP and the resulting current state is DOWN. Subrequisite checking occurs when the desired or current state of a resource changes, and the resulting desired state is DOWN and the resulting current state is UP. The terms UP and DOWN refer to table-relative up and down states, which are defined in the directory table.
If the desired and current states match each other (both are UP or both are DOWN), prerequisite and subrequisite checking do not occur. They also do not occur if either state fails to match one of the table-relative UP or DOWN states. Table-relative states are considered prefixes when compared to the desired state and current state to determine a match. For example, a desired state of UPWAIT matches a table-relative state of UP because UP is considered a prefix, and it matches the prefix UP in UPWAIT.
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