Wan Load Balance feature designed to load balancing outgoing traffic over its member interfaces per ECMP and load balancing algorithm. so usually a static/default required to configured over wan-load-balance/virtual-wan-link to make ECMP happened.
If another default route allowed, may mean one more ECMP route added, FGT will face the dilemma of load balancing between Wan Load Balance member interfaces or with the one more interface.
if you just want add one more backup interface, you can put it into wan load balance with different distance or priority setting
I am having the same issue. I need to add a second default route for failover purposes. The second default route will be at a higher distance. I'm using the spillover functionality on the WLB, so I would prefer to keep that functionality. I'm surprised Fortinet hasn't fixed this by now.
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