OSPF Adjacency Troubleshooting: Point-to-Point Link Up but No Neighbor

Answer analysis

Option-by-option breakdown

For each option: why learners choose it and why it is or isn't the right answer here.

• ✗

  The routers are in different OSPF areas

  Why it's wrong here

  OSPF requires all routers on a shared link to belong to the same area. If one router is configured with area 0 and the other with area 1, the Area ID field in the hello packet does not match, so the receiving router discards the packet before any neighbor state is formed. This definitely prevents adjacency, but it is less common on a simple two-router point-to-point Ethernet link than a hello/dead interval mismatch, which is the more typical configuration error.

• ✗

  OSPF authentication is configured on one router but not the other

  Why it's wrong here

  When OSPF authentication is enabled on an interface, every hello packet carries an authentication type and key in the OSPF header. If one router is configured with MD5 authentication (type 2) or a plain-text password (type 1), and the other router expects no authentication (type 0), each side rejects the received hello because the authentication fields do not match local expectations. This mismatch produces explicit error messages such as 'OSPF packet authentication failed,' so it is not a silent failure like the interval mismatch—making it a distinct and less likely cause in this scenario.

• ✗

  The routers have the same router ID

  Why it's wrong here

  A router ID is a unique 32-bit identifier, typically derived from the highest loopback address. If two directly connected routers are configured with the same router ID, each sees the other's hello advertising its own RID, causing the routing process to regard the neighbor as invalid and refuse to establish an adjacency. Duplicate RIDs can also lead to routing instability and blackholes, but in a straightforward two-router setup, this is an uncommon misconfiguration; a hello/dead interval mismatch is far more frequent and is the correct answer here.

• ✓

  The OSPF hello and dead intervals are mismatched

  Why this is correct

  OSPF routers must use identical hello and dead intervals on a given interface because these values are advertised inside every hello packet. When the receiving router compares the peer's hello interval or dead interval with its own configured values and they differ, it silently drops the hello without generating an error log, so the neighbor never advances past the Down state. This is a classic configuration error, especially when timers are adjusted on only one end of the link, and it directly matches the scenario where two connected routers with otherwise valid OSPF settings fail to become adjacent.

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