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Introduction:
When a monitored interface in HA cluster goes down it triggers a failover for the cluster members. When the monitored interface experience a flapping up/down behavior this in turn will trigger frequent failovers among the cluster members causing instability. This in turn may affects the system resources such a memory, CPU, etc., especially if session-pickup setting is enabled.
Scenario:
Here, it is assumed that the FortiGate is configured as follows. System link-monitor configured to use wan1 to ping an external server, for example 8.8.8.8.
config system link-monitor
edit "wan1-ping-server"
set srcintf "wan1"
set server "8.8.8.8"
set update-cascade-interface enable
set update-static-route enable
next
end
Upon checking the system link-monitor (diagnose system link-monitor status), it is observed that the status is flapping between alive/dead. This indicates there is a reachability problem, which may be due to an ISP issue or any intermediate router routing issues in the path to the destination.
The system HA cluster configured as per the following (port1 is the monitored interface):
config system ha
set group-name "FGT-HA"
set mode a-p
set monitor "port1"
end
Need to relate the flapping behavior of the system link-monitor interface, wan1, with the repeated failovers in the system HA cluster.
Root Cause:
Need to check the configuration of the system interface settings for the source interface in system link-monitor, i.e., wan1. Need to confirm if the fail-detect setting is enabled and which system interface it's connected to using fail-alert-interfaces setting. As it's evident from the following configuration, it was noticed that the system link-monitor is the source of the problem as it triggers the flapping behavior on the monitored interface under system HA cluster configuration:
config system interface
edit "wan1"
set ip 192.168.1.254 255.255.255.0
set fail-detect enable
set fail-detect-option detectserver link-down
set fail-alert-method link-down
set fail-alert-interfaces "port1"
next
end
Note: There may be other causes that trigger the flapping behavior for the system HA cluster units. This article shows only one possible root cause.
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