Frequent HA switching after setting link-monitor

[VDOM001]

We have been using link-monitor to monitor ping to GW, but after setting up link-monitor, HA switchover due to link monitor failure occurred frequently during periods of high traffic spikes.
When we checked the ping statistics of link-monitor, we found a temporary maximum latency of 475 ms, which is a very bad value. No switching occurred at that time.
The reason for the frequent occurrences is that the line became unstable due to user traffic.
Is it safe to assume that the ICMP packets in the link monitor are likely to have been affected by the unstable state of the line due to user traffic?
If so, can this be resolved by changing the timer value of the link monitor?
The current settings are interval 5000, failtime 3, and other default values.

[VDOM001]

@funkylicious 

It is easy to monitor the SVI of the L3SW since the FW needs to be switched in case of failure between L2SW and L3SW.

[funkylicious]

ok, so the issue is in fact monitoring the SVI of L3SW which is causing issues.

is this a remote device that's outside your LAN to which you have a 10Mbps connection ?

[VDOM001]

@funkylicious 

The L3SW is located inside the LAN, not outside.
(lan)Forti(wan)----L2SW----L3SW

[funkylicious]

well, the issue is clearly regarding icmp packets being lost along the way due to either congestion or something else.

if the L3SW are individual and not stacked, I assume you run some kind of L3 redundancy protocol on them, like VRRP/HSRP/GLSB so if one fails traffic would be handled by the other ?

if so, L2SW's are or should be connected in both L3SW to ensure L2 redundancy ( leave layer2 things like loops for STP to handle ) and I would not think that link monitor should be used since the traffic from FGT would have the nexthop a virtual ip.

if L2SW's fails, the interface should be already monitored by HA and failover should happen automatically.

like this:

L.E. if L2SW's are not stack-able but individual, i would also consider redundant interface from the FGT to each L2SW.

just my thoughts.

[VDOM001]

@funkylicious 

Thank you very much.

As you mentioned, the L3SW is redundant with HSRP, so the L2SW must be connected to both L3SWs as shown in the figure.
Once we make minor modifications to the link monitor parameters and if the problem is not resolved, we will consider including a proposal to configure L2 redundancy with STP and not use link monitor.

[Toshi_Esumi]

That means the link-monitor is not proper way (meaningless) to detect the primary unit's operation problem to trigger HA failover. Just monitoring ethernet link without link-monitor is more meaningful way to trigger HA failover.

Toshi