The answer is an MTU mismatch between the two routers. When OSPF neighbors are stuck in the EXSTART state, the routers have progressed past the two-way and init states but cannot agree on which router will be the master for Database Description (DBD) packet exchange. If the interface MTU values differ, the router with the smaller MTU will drop any DBD packet that exceeds its size limit, preventing the master/slave negotiation from completing and blocking all LSA exchange. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your ability to look beyond common misconfigurations like mismatched areas or authentication—MTU is a subtle but classic cause of EXSTART stalling. A common trap is assuming the issue is a Layer 1 or 2 problem, but the neighbor state itself points directly to the OSPF packet sizing issue. Memory tip: EXSTART = EXcessive Size Trouble, so check the MTU first.
300-410 Network Logging and Syslog Practice Question
This 300-410 practice question tests your understanding of network logging and syslog. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. After answering, compare your reasoning against the explanation and wrong-answer breakdown below. Once you have made your selection, read the full explanation to reinforce the concept and understand why each distractor is designed to mislead on exam day.
An engineer configures OSPF on two routers connected via a serial link. Both routers show the neighbor state as EXSTART/EXSTART, and no LSAs are exchanged. The engineer verifies that the OSPF process IDs are the same, areas match, and authentication is correct. Which is the most likely explanation?
Clue words in this question
Noticing these words before you look at the options changes how you read each choice.
Clue: "most likely"
Why it matters: Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
MTU mismatch between the two routers
In OSPF, when neighbors are stuck in EXSTART, it often indicates an MTU mismatch. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU differs, the receiving router will reject DBD packets larger than its own MTU, causing the neighbor to remain in EXSTART. This is a common edge case because MTU is not always checked during initial troubleshooting.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
✓
MTU mismatch between the two routers
Why this is correct
Correct. An MTU mismatch causes DBD packets to be dropped, preventing the neighbor from advancing past EXSTART.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
OSPF network type mismatch (e.g., point-to-point vs broadcast)
Why it's wrong here
A network type mismatch typically causes the neighbor to get stuck in INIT or 2WAY, not EXSTART.
✗
OSPF hello interval mismatch
Why it's wrong here
A hello interval mismatch prevents the neighbor from forming at all; the state would remain DOWN.
✗
OSPF dead interval mismatch
Why it's wrong here
A dead interval mismatch also prevents neighbor formation, resulting in DOWN state.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
OSPF neighbours must agree on key parameters.
Router ID selection can affect neighbour relationships and LSDB output.
OSPF cost influences the preferred path.
A route can appear in OSPF information but not become the installed route.
TExam Day Tips
→Check area mismatch first when OSPF adjacency fails.
→Review passive interfaces when a network is advertised but no neighbour forms.
→Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Real-world example
How this comes up in practice
A network engineer at a university connects two campus buildings via a fibre link. Both routers run OSPF, but no adjacency forms — even though both routers can ping each other. The engineer finds one router is in area 0 and the other in area 1. OSPF adjacency requires matching area numbers, hello/dead timers, and network type. IP reachability alone is not enough.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Network Logging and Syslog — This question tests Network Logging and Syslog — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: MTU mismatch between the two routers — In OSPF, when neighbors are stuck in EXSTART, it often indicates an MTU mismatch. OSPF uses the interface MTU to determine the size of Database Description (DBD) packets. If the MTU differs, the receiving router will reject DBD packets larger than its own MTU, causing the neighbor to remain in EXSTART. This is a common edge case because MTU is not always checked during initial troubleshooting.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
Are there clue words in this question I should notice?
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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