OSPF Adjacency: Troubleshooting Mismatched Area ID, Timers, and MTU
A network engineer is troubleshooting an OSPF adjacency that fails to reach FULL state between two Nexus 9000 switches. Which TWO are common causes for this issue?
Quick Answer
The answer is mismatched hello/dead timers and mismatched area IDs. OSPF adjacency requires that both routers agree on the same hello and dead intervals, as well as the same area ID, because these parameters are included in the Hello packet and must match for neighbor discovery and state progression; if either differs, the routers will remain stuck in the INIT or EXSTART/EXCHANGE state and never reach FULL. On the Cisco DCCOR 350-601 exam, this concept tests your understanding of OSPF neighbor formation fundamentals on Nexus 9000 switches, often appearing as a multi-select troubleshooting scenario where you must identify two common causes from a list that includes MTU mismatch and network type mismatch as distractors. A common trap is assuming that only one parameter can block adjacency, but the exam expects you to recognize that both timer and area mismatches are equally frequent culprits in data center environments. Remember the memory tip: “Timers and areas must match—if either is off, the adjacency will catch.”
⚠ Common exam trap
Cisco often tests the misconception that the OSPF process ID must match between neighbors, but it is only locally significant and does not affect adjacency formation.
Answer choices
Why each option matters
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
Mismatched area ID
OSPF requires that both routers in a neighbor relationship belong to the same area. If the area IDs differ, the routers will not exchange routing information and the adjacency will stall at the EXSTART or EXCHANGE state, never reaching FULL. This is a fundamental OSPF neighbor requirement defined in RFC 2328.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
Mismatched OSPF process ID
Why it's wrong here
OSPF process ID is local to each router and does not affect adjacency formation.
- ✓
Mismatched area ID
Why this is correct
OSPF neighbors must belong to the same area to form an adjacency.
- ✗
Mismatched router ID
Why it's wrong here
Router IDs must be unique, but a mismatch does not prevent adjacency as long as they are unique.
- ✓
Mismatched hello/dead timers
Why this is correct
Hello and dead timers must match for OSPF neighbors to form adjacency.
- ✗
Mismatched network type
Why it's wrong here
Network type mismatch does not prevent adjacency; timers can be adjusted to match.
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Same concept, more angles
1 more way this is tested on 350-601
These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.
Variation 1. A network engineer is troubleshooting an OSPF adjacency issue between two Nexus switches. The neighbors are stuck in the EXSTART state. What is the most likely cause?
medium- A.Hold timer mismatch
- B.Incorrect area ID
- ✓ C.MTU mismatch
- D.Duplicate router ID
Why C: When OSPF neighbors are stuck in the EXSTART state, the most common cause is an MTU mismatch. In EXSTART, the routers exchange Database Description (DBD) packets, which must be no larger than the interface MTU. If one interface has a smaller MTU, the DBD packet is dropped, preventing the adjacency from progressing to the Exchange state. Cisco Nexus switches default to an MTU of 1500 bytes, but any mismatch will cause this issue.
JA
Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
This 350-601 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-601 exam.