Core Issue
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.
Resolution
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
- Forwarding Address Known via an External Route
- Distribute List Is Blocking the Routes
For indepth information, refer to Why Are Some OSPF Routes in the Database but Not in the Routing Table?
Type of Route Missing from Routing Table
External routes only
