There have been questions often asked on difference between Availability and Reachability rules. Following articles describes them both.
Availability and Reachability Rules
Availability and Reachability are important special cases in the Live Exceptions rule definitions.
- Reachability is the ability of the CA eHealth poller to communicate with the device that contains an element. To be reachable, a device must respond to ICMP pings. On each poll of the device, the CA eHealth poller indicates to Live Exceptions whether a device is reachable.
- Standard Reachability Rule
- Specifies that an alarm will be generated as soon as an element is unreachable.
- Time Over Threshold for Reachability
Specifies a time threshold which must be crossed before a reachability rule trips an alarm. This lets you create reachability rules that account for planned downtime or for fairly dynamic environments where some unscheduled downtime is not considered a reason to generate reachability alarms.
- Availability is more complex than reachability; its definition is time- dependent. The CA eHealth poller assesses properties of the device such as reboots through SNMP variables such as sysUpTime and ifOperStatus or equivalent variables, when defined by the device. Availability is generally not known by the CA eHealth poller until it successfully polls the device, so an immediate value is not always obtainable on each poll. There are two types of availability rules which are described as follows:
- Standard Availability Rule
Specifies that an alarm will be generated as soon as unavailability is recognized. The alarm clears when the element is reachable for a number of minutes defined by the user.
- Time Over Threshold for Availability
Specifies that an alarm will be generated when a device is unavailable for a defined number of minutes out of a total number of hours. The number of minutes and the number of hours are both user-defined. The alarm clears when the condition is no longer true.
Note: The default condition is set to below and the default threshold is set to 100% and cannot be changed.
You specify Reachability and Availability alarms by defining an analysis window. A non-reachable device generates an alarm as soon as the poller is unable to reach the device. An unavailable device generates an alarm when the CA eHealth poller has a successful poll during which availability can be determined; this may be some time after the device was unavailable. Special processing by Live Exceptions ensures, however, that an alarm is generated, and thus even transient periods of unavailability are eventually reported. However, you cannot compensate for potential reporting delay. If you want immediate notification of potential problems, you should also define a Reachability alarm for those situations.