In most cases, Open Shortest Path First (OSPF) finds a discrepancy in the database, so it does not install the route in the routing table. Often, you will receive the Adv Router is not-reachable error message, indicating that the router advertising the Link-State Advertisement (LSA) is not reachable through OSPF on top of the LSA in the database when this problem occurs.
There are several possible reasons for this problem, most of which deal with mis-configuration or a broken topology. When the configuration is corrected, the OSPF database discrepancy goes away, and the routes appear in the routing table.
Some of the reasons include;
Network Type Mismatch
Wrong Address Assignment in DUAL Serial Link Setup
One Side of Point-to-Point Link Included in Wrong Majornet or Subnet
One Side Is Unnumbered and the Other Side Is Numbered
Broken PVC in Fully Meshed Frame Relay Environment