Several configuration parameters define how Zabbix server should behave when an agent check (Zabbix, SNMP, IPMI, JMX) fails and a host interface becomes unreachable.
A host interface is treated as unreachable after a failed check (network error, timeout) by Zabbix, SNMP, IPMI or JMX agents. Note that Zabbix agent active checks do not influence interface availability in any way.
From that moment UnreachableDelay defines how often an interface is rechecked using one of the items (including LLD rules) in this unreachability situation and such rechecks will be performed already by unreachable pollers (or IPMI pollers for IPMI checks). By default it is 15 seconds before the next check.
In the Zabbix server log unreachability is indicated by messages like these:
Zabbix agent item "system.cpu.load[percpu,avg1]" on host "New host" failed: first network error, wait for 15 seconds Zabbix agent item "system.cpu.load[percpu,avg15]" on host "New host" failed: another network error, wait for 15 seconds
Note that the exact item that failed is indicated and the item type (Zabbix agent).
The UnreachablePeriod parameter defines how long the unreachability period is in total. By default UnreachablePeriod is 45 seconds. UnreachablePeriod should be several times bigger than UnreachableDelay, so that an interface is rechecked more than once before an interface becomes unavailable.
When the unreachability period is over, the interface is polled again, decreasing priority for item that turned the interface into unreachable state. If the unreachable interface reappears, the monitoring returns to normal automatically:
resuming Zabbix agent checks on host "New host": connection restored
So, after the interface becomes available, items are not polled immediately, but they are getting rescheduled to their next polling round.
After the UnreachablePeriod ends and the interface has not reappeared, the interface is treated as unavailable.
In the server log it is indicated by messages like these:
temporarily disabling Zabbix agent checks on host "New host": interface unavailable
and in the frontend the host availability icon goes from green/gray to yellow/red (the unreachable interface details can be seen in the hint box that is displayed when a mouse is positioned on the host availability icon):
The UnavailableDelay parameter defines how often an interface is checked during interface unavailability.
By default it is 60 seconds (so in this case “temporarily disabling”, from the log message above, will mean disabling checks for one minute).
When the connection to the interface is restored, the monitoring returns to normal automatically, too:
enabling Zabbix agent checks on host "New host": interface became available