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When using the ECMP routing failover feature with a wireless modem as the secondary interface, the routing to the modem interface will not get enabled automatically, even when the primary interface is down and the modem interface is up. This would happen because when a dynamic interface comes up, the routing table will not be updated automatically.
FortiOS version 4.00 MR3
FortiOS version 5.0.x
For this to work, it is necessary to enable a 'dynamic-gateway' option on the modem interface route:
config router static edit <seq_no> dynamic-gateway enable next end
Once that is done, routing entries will be updated properly when a PPPoE dynamic interface comes up.