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Page 254 7750 SR OS Router Configuration Guide
The host-unreachable command can reference any valid local or remote IP address. The ability to
ARP a local IP address or find a remote IP address within a route prefix in the route table is
considered part of the monitoring procedure. The host-unreachable priority event operational state
tracks ARP or route table entries dynamically appearing and disappearing from the system. The
operational state of the host-unreachable event can be one of the following:
Unlike other priority event types, the host-unreachable priority event monitors a repetitive task. A
historical evaluation is performed on the success rate of receiving ICMP echo reply messages. The
operational state takes its cleared and set orientation from the historical success rate. The
informational portion of the operational state is derived from the last attempt’s result. It is possible
for the previous attempt to fail while the operational state is still cleared due to an insufficient number
of failures to cause it to become set. It is also possible for the state to be set while the previous
attempt was successful.
When an event transitions from clear to set, the set is processed immediately and must be reflected in
the associated virtual router instances in-use priority value. As the event transitions from clear to set,
a hold set timer is loaded with the value configured by the events hold-set command. This timer
prevents the event from clearing until it expires, damping the effect of event flapping. If the event
clears and becomes set again before the hold set timer expires, the timer is reset to the hold-set value,
extending the time before another clear can take effect.
The hold-set timer be expired and the historical success rate must be met prior to the event
operational state becoming cleared.
Host Unreachable
Operational State
Description
Set – no ARP No ARP address found for ip-addr for drop-count consecutive
attempts. Only applies when IP address is considered local.
Set – no route No route exists for ip-addr for drop-count consecutive attempts. Only
when IP address is considered remote.
Set – host unreachable ICMP host unreachable message received for drop-count consecutive
attempts.
Set – no reply ICMP echo request timed out for drop-count consecutive attempts.
Set – reply received Last ICMP echo request attempt received an echo reply but historically
not able to clear the event.
Cleared – no ARP No ARP address found for ip-addr - not enough failed attempts to set
the event.
Cleared – no route No route exists for ip-addr - not enough failed attempts to set the event.
Cleared – host
unreachable
ICMP host unreachable message received - not enough failed attempts
to set the event.
Cleared – no reply ICMP echo request timed out - not enough failed attempts to set the
event.
Cleared – reply
received
Event is cleared - last ICMP echo request received an echo reply.