Device Availability and Reachability

Availability measures system uptime and reachability measures device connectivity to capm.
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Availability measures system uptime and reachability measures device connectivity to 
CA Performance Management
.
Availability 
is a percentage of time that the device is powered on and capable of processing data. 
CA Performance Management
 uses system uptime to calculate the percentage of time within a poll cycle that the device was available. The metric value is recorded as up (100), or down (0), or a percentage of the poll cycle.
If 
CA Performance Management
 is unable to reach a device for a particular timeframe, availability is backfilled when the device sends a poll response. 
CA Performance Management
 uses the system uptime to calculate which poll cycles to mark as available. If the device was restarted, or if a counter rollover for uptime occurred, 
CA Performance Management
 backfills only from the 0 to the current system uptime value. Poll cycles before the rollover or restart, where no poll responses were received, remain blank. Hourly and daily rollups reflect the backfill data only if the data is received before the rollup occurs. Availability is the only metric that 
CA Performance Management
 backfills.
A device that is available might be unreachable because of a network or communications failure by another device.
Reachability 
refers to whether a device is reachable from the data source. Typically, data sources use ICMP (ping testing) to communicate regularly with the target device. Any communication failures, including the loss of the network path or routing, affect the reachability statistics. If ICMP is blocked, 
CA Performance Management
 uses SNMP to determine reachability.
Reachability data comes from regular ping testing of all devices that support ICMP. The reachability value is the percentage ping responses that are received from the device during each reporting interval.