Passive interface problems
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 This will result in the passive OSPF FortiGate unit interface having an incomplete or out-of-date link-state database, and it will not be able to properly route its traffic.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.
Timer problems
A timer mismatch is when two routers have different values set for the same timer. For example if one router declares a router dead after 45 seconds and another waits for 4 minutes that difference in time will result in those two routers being out of synch for that period of time—one will still see that offline router as being online.
The easiest method to check the timers is to check the configuration on each router. Another method is to sniff some packets, and read the timer values in the packets themselves from different routers. Each packet contains the hello interval, and dead interval periods, so you can compare them easily enough.
Bi–directional Forwarding Detection (BFD)
Bi-directional Forwarding Detection (BFD) is a protocol used to quickly locate hardware failures in the network. Routers running BFD communicate with each other, and if a timer runs out on a connection then that router is declared down. BFD then communicates this information to the routing protocol and the routing information is updated.
Authentication issues
OSPF has a number of authentication methods you can choose from. You may encounter problems with routers not authenticating as you expect. This will likely appear simply as one or more routers that have a blind spot in their routing – they won’t acknowledge a router. This can be a problem if that router connects areas to the backbone as it will appear to be offline and unusable.
To confirm this is the issue, the easiest method is to turn off authentication on the neighboring routers. With no authentication between any routers, everything should flow normally.
Another method to confirm that authentication is the problem is to sniff packets, and look at their contents. The authentication type and password are right in the packets which makes it easy to confirm they are what you expect during real time. Its possible one or more routers is not configured as you expect and may be using the wrong authentication. This method is especially useful if there are a group of routers with these problems—it may only be one router causing the problem that is seen in multiple routers.
Once you have confirmed the problem is authentication related, you can decide how to handle it. You can turn off authentication and take your time to determine how to get your preferred authentication type back online. You can try another type of authentication, such as text instead of md5, which may have more success and still provide some level of protection. The important part is that once you confirm the problem, you can decide how to fix it properly.