A passive OSPF interface doesn’t send out any updates. This means it can’t be a DR, BDR, or an area border router among other things. It will depend on other neighbor routers to update its link-state table.

Passive interfaces can cause problems when they aren’t receiving the routing updates you expect from their neighbors. This will result in the passive OSPF FortiGate unit interface having an incomplete or out-of-date link-state database, and it will not be able to properly route its traffic. It is possible that the passive interface is causing a hole in the network where no routers are passing updates to each other, however this is a rare situation.

If a passive interface is causing problems, there are simple methods to determine it is the cause. The easiest method is to make it an active interface, and if the issues disappear, then that was the cause. Another method is to examine the OSPF routing table and related information to see if it is incomplete compared to other neighbor routers. If this is the case, you can clear the routing table, reset the device and allow it to repopulate the table.

If you cannot make the interface active for some reason, you will have to change your network to fix the “hole” by adding more routers, or changing the relationship between the passive router’s neighbors to provide better coverage.