N10-009 Network Troubleshooting Practice Question
Two routers are configured with OSPF in the same area, but they do not form an adjacency. Router A shows OSPF state EXSTART, and Router B shows state EXSTART. Which of the following is the most likely cause?
⚠ Common exam trap
CompTIA often tests the MTU mismatch trap by having candidates confuse it with Hello/Dead interval mismatches, but the key clue is that both routers are stuck in EXSTART, not in INIT or 2-Way.
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
✓
The MTU is mismatched between the two routers
When OSPF routers are stuck in the EXSTART state, it indicates that they have progressed past the 2-Way and ExStart phases but are unable to exchange Database Description (DBD) packets. The most common cause is an MTU mismatch, because OSPF will not proceed to the Exchange state if a DBD packet exceeds the interface MTU of the neighbor. This causes the routers to continuously renegotiate the master/slave relationship without completing the exchange.
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 OSPF Hello and Dead intervals are mismatched
Why it's wrong here
OSPF routers exchange Hello packets to discover neighbors and maintain adjacency. For a neighbor relationship to progress to the Two-Way state, the Hello and Dead intervals configured on the interfaces must precisely match. If these timers are mismatched, routers will receive Hello packets but will not acknowledge them as valid, preventing them from seeing each other in their respective neighbor lists. This condition typically causes the adjacency to remain stuck in the Init state, failing to advance to Two-Way or subsequent states like ExStart.
When this WOULD be correct
A question where routers show OSPF state INIT or DOWN, or where they never progress past the 2-WAY state, would make Hello/Dead interval mismatch the correct answer. For example: 'Two OSPF routers in the same area are configured with different Hello intervals. Which state will they remain stuck in?'
- ✗
The OSPF area IDs are different
Why it's wrong here
OSPF routers must belong to the same area to establish a neighbor relationship. If the OSPF area IDs are different on the interfaces attempting to form an adjacency, the routers will not even recognize each other as potential neighbors. This mismatch causes the routers to drop each other's Hello packets, preventing them from progressing beyond the Down state and certainly not reaching the ExStart state. Adjacency formation is fundamentally dependent on matching area IDs.
When this WOULD be correct
This option would be correct in a scenario where two OSPF routers are configured with different area IDs (e.g., one in area 0 and the other in area 1) and fail to form an adjacency, with states stuck at INIT or 2-WAY, not EXSTART.
- ✓
The MTU is mismatched between the two routers
Why this is correct
During the OSPF ExStart state, routers exchange Database Description (DBD) packets to negotiate the master/slave relationship and synchronize their link-state databases. If the Maximum Transmission Unit (MTU) values on the connected interfaces are mismatched, one router might send a DBD packet larger than the other router's configured MTU. The receiving router will then drop these oversized packets, preventing the successful exchange and acknowledgment of DBDs, thereby causing the adjacency to remain perpetually stuck in the ExStart state. This critical mismatch prevents progression to the Exchange state.
- ✗
The network type is misconfigured (e.g., one side is broadcast, the other is point-to-point)
Why it's wrong here
A mismatch in OSPF network types, such as one interface configured for broadcast and the other for point-to-point, can lead to significant issues in neighbor discovery and Designated Router (DR)/Backup Designated Router (BDR) election processes. While this misconfiguration can prevent full adjacency, it typically allows the routers to progress beyond the ExStart state, often getting stuck in the Exchange or Loading states due to problems with DR election or LSA exchange mechanisms. The fundamental neighbor discovery and initial DBD exchange usually succeed, unlike an MTU mismatch.
When this WOULD be correct
If the question described routers stuck in the INIT or 2-WAY state, or failing to form an adjacency entirely, a network type mismatch (e.g., one side broadcast, the other point-to-point) would be the most likely cause, as it affects Hello packet handling and neighbor discovery.
Option-by-option analysis
Why each answer is right or wrong
Understanding why wrong answers are wrong — and when they would be correct — is what separates a 750 score from a 900. The N10-009 exam frequently reuses these exact scenarios with slightly different constraints.
✓The MTU is mismatched between the two routersCorrect answer▾
Why this is correct
During the OSPF ExStart state, routers exchange Database Description (DBD) packets to negotiate the master/slave relationship and synchronize their link-state databases. If the Maximum Transmission Unit (MTU) values on the connected interfaces are mismatched, one router might send a DBD packet larger than the other router's configured MTU. The receiving router will then drop these oversized packets, preventing the successful exchange and acknowledgment of DBDs, thereby causing the adjacency to remain perpetually stuck in the ExStart state. This critical mismatch prevents progression to the Exchange state.
✗The OSPF Hello and Dead intervals are mismatchedWrong answer — click to see why▾
Why this is wrong here
In this scenario, both routers are stuck in EXSTART state, which indicates a problem with the Database Description (DD) packet exchange. MTU mismatch is the classic cause of this issue, whereas Hello/Dead interval mismatches typically prevent the routers from reaching the 2-WAY state, not EXSTART.
★ When this WOULD be the correct answer
A question where routers show OSPF state INIT or DOWN, or where they never progress past the 2-WAY state, would make Hello/Dead interval mismatch the correct answer. For example: 'Two OSPF routers in the same area are configured with different Hello intervals. Which state will they remain stuck in?'
Why candidates choose this
Candidates often confuse the symptoms of different OSPF issues. Since Hello/Dead intervals are a common misconfiguration, they assume it can cause any adjacency problem, including the EXSTART state, without understanding the specific state progression.
✗The OSPF area IDs are differentWrong answer — click to see why▾
Why this is wrong here
In OSPF, mismatched area IDs prevent routers from forming an adjacency because they would be in different areas, but the question states both routers are in the same area. The EXSTART state indicates they have passed the 2-way state, which requires matching area IDs, so area ID mismatch cannot be the cause.
★ When this WOULD be the correct answer
This option would be correct in a scenario where two OSPF routers are configured with different area IDs (e.g., one in area 0 and the other in area 1) and fail to form an adjacency, with states stuck at INIT or 2-WAY, not EXSTART.
Why candidates choose this
Candidates know that OSPF requires matching area IDs for adjacency, so they may jump to this answer without noticing that the EXSTART state implies area IDs already matched.
✗The network type is misconfigured (e.g., one side is broadcast, the other is point-to-point)Wrong answer — click to see why▾
Why this is wrong here
In this scenario, both routers are stuck in EXSTART, which indicates a problem during the database description (DD) packet exchange. A network type mismatch typically prevents the routers from reaching the 2-WAY state, not EXSTART.
★ When this WOULD be the correct answer
If the question described routers stuck in the INIT or 2-WAY state, or failing to form an adjacency entirely, a network type mismatch (e.g., one side broadcast, the other point-to-point) would be the most likely cause, as it affects Hello packet handling and neighbor discovery.
Why candidates choose this
Candidates may confuse the symptoms of network type mismatch (failure to reach 2-WAY) with those of MTU mismatch (stuck in EXSTART), as both involve OSPF adjacency formation issues.
Analysis generated from the official N10-009blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
Visual reference
Quick reference
Routing Protocol Comparison
| Protocol | Metric | Max Hops | Algorithm | Type |
|---|---|---|---|---|
| RIP v2 | Hop count | 15 | Bellman-Ford | Distance vector |
| OSPF | Cost (bandwidth) | Unlimited | Dijkstra (SPF) | Link state |
| EIGRP | Composite metric | Unlimited | DUAL | Hybrid |
| IS-IS | Cost | Unlimited | Dijkstra | Link state |
| BGP | Policy / attributes | Unlimited | Path vector | Path vector |
RIP's 15-hop limit makes it unsuitable for large networks. OSPF and EIGRP dominate modern enterprise deployments.
Go deeper
Related to this question
Learn chapter
Interface Error Types: CRC, Runts, Giants
Key term
OSPF
OSPF is a link-state routing protocol that uses the SPF algorithm to compute the shortest path to each destination within a single autonomous system.
Key term
MTU
MTU (Maximum Transmission Unit) is the largest size of a single data packet that can be sent over a network without needing to be fragmented.
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Written by Johnson Ajibi, MSc IT Security
Senior Network & Security Engineer · founder of Courseiva
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