- A
Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked.
This is correct because OSPF multicast hellos (224.0.0.5) must be permitted through the firewall for neighbors to form. If blocked, routers will not see each other's hellos, resulting in an empty neighbor table.
- B
Verify that the OSPF router IDs on R1 and R2 are unique.
Why wrong: While duplicate router IDs can cause issues in OSPF, they would not prevent the initial hello exchange or the appearance of a neighbor in the neighbor table (even if adjacency fails). The neighbor would still be seen, possibly cycling states. This step is too deep given the complete absence of neighbors.
- C
Check the OSPF network type on the connecting interfaces.
Why wrong: Mismatched network types can prevent adjacency (e.g., broadcast vs. point-to-point), but interface timers have already been verified as matching. A mismatch would typically still show a neighbor stuck in 2-way or other state, not a total lack of neighbors. Moreover, the firewall in the path makes multicast filtering a more probable and easier-to-verify cause.
- D
Verify that the MTU on the connecting interfaces matches.
Why wrong: MTU mismatch usually causes neighbors to get stuck in the EXSTART/EXCHANGE state because DBD packets cannot be fully exchanged. The routers would still discover each other via hellos and appear in the neighbor table. Therefore, checking MTU does not address why hellos are not showing up at all.
Quick Answer
The answer is to check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked. This is correct because OSPF relies on multicast Hello packets sent to 224.0.0.5 for neighbor discovery on all network types except non-broadcast; if a firewall in the path filters this multicast traffic, the routers will never see each other’s Hellos, resulting in an empty neighbor table even when all interface parameters like area, timers, and subnet match perfectly. On the CCNA 200-301 v2 exam, this scenario tests your understanding of OSPF’s reliance on Layer 3 multicast versus common Layer 4 filtering, and a frequent trap is diving into OSPF-specific debug commands or authentication checks before verifying basic firewall rules. Remember the memory tip: "No neighbors? Check the firewall for 224.0.0.5 first—OSPF can’t shake hands if the multicast is banned."
CCNA IP Routing Practice Question
This 200-301 practice question tests your understanding of ip routing. 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.
A network engineer configures OSPF on R1 and R2 over a point-to-point link. The interfaces are in the same OSPF area, with matching hello and dead timers, and are on the same IP subnet. However, the command show ip ospf neighbor on both routers shows no neighbors. A firewall sits between R1 and R2. What should the technician do next?
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
Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked.
OSPF uses multicast address 224.0.0.5 for hello packets on all OSPF network types except non-broadcast. With a firewall in the path, the most common reason for a complete lack of neighbor discovery, even when the interface-level OSPF parameters match, is that the firewall is blocking this multicast traffic. Checking the firewall rules directly addresses the most likely layer-3/layer-4 filtering issue and is a non-intrusive first step before examining deeper OSPF-specific settings.
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.
- ✓
Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked.
- ✗
Verify that the OSPF router IDs on R1 and R2 are unique.
Why it's wrong here
While duplicate router IDs can cause issues in OSPF, they would not prevent the initial hello exchange or the appearance of a neighbor in the neighbor table (even if adjacency fails). The neighbor would still be seen, possibly cycling states. This step is too deep given the complete absence of neighbors.
- ✗
Check the OSPF network type on the connecting interfaces.
Why it's wrong here
Mismatched network types can prevent adjacency (e.g., broadcast vs. point-to-point), but interface timers have already been verified as matching. A mismatch would typically still show a neighbor stuck in 2-way or other state, not a total lack of neighbors. Moreover, the firewall in the path makes multicast filtering a more probable and easier-to-verify cause.
- ✗
Verify that the MTU on the connecting interfaces matches.
Why it's wrong here
MTU mismatch usually causes neighbors to get stuck in the EXSTART/EXCHANGE state because DBD packets cannot be fully exchanged. The routers would still discover each other via hellos and appear in the neighbor table. Therefore, checking MTU does not address why hellos are not showing up at all.
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 200-301 exam frequently reuses these exact scenarios with slightly different constraints.
✓Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked.Correct answer▾
Why this is correct
This is correct because OSPF multicast hellos (224.0.0.5) must be permitted through the firewall for neighbors to form. If blocked, routers will not see each other's hellos, resulting in an empty neighbor table.
✗Verify that the OSPF router IDs on R1 and R2 are unique.Wrong answer — click to see why▾
Why this is wrong here
Router ID duplication does not explain a completely empty neighbor table; neighbors would still be detected via hellos.
✗Check the OSPF network type on the connecting interfaces.Wrong answer — click to see why▾
Why this is wrong here
Network type mismatch would likely still result in hellos being received and a neighbor entry appearing, just not advancing to full adjacency. The complete absence of neighbors points to a transport issue.
✗Verify that the MTU on the connecting interfaces matches.Wrong answer — click to see why▾
Why this is wrong here
Candidates often remember MTU mismatch as a frequent OSPF problem, but it manifests after neighbor discovery, not as a complete lack of neighbors.
Analysis generated from the official 200-301blueprint and verified against question context. The “when correct” sections are what AI assistants cite when candidates ask “what’s the difference between these options?”
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.
Trap categories for this question
Command / output trap
Mismatched network types can prevent adjacency (e.g., broadcast vs. point-to-point), but interface timers have already been verified as matching. A mismatch would typically still show a neighbor stuck in 2-way or other state, not a total lack of neighbors. Moreover, the firewall in the path makes multicast filtering a more probable and easier-to-verify cause.
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.
What to study next
Got this wrong? Here's your next step.
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 200-301 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 200-301 question test?
IP Routing — This question tests IP Routing — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Check the firewall rules to verify that multicast address 224.0.0.5 is not being blocked. — OSPF uses multicast address 224.0.0.5 for hello packets on all OSPF network types except non-broadcast. With a firewall in the path, the most common reason for a complete lack of neighbor discovery, even when the interface-level OSPF parameters match, is that the firewall is blocking this multicast traffic. Checking the firewall rules directly addresses the most likely layer-3/layer-4 filtering issue and is a non-intrusive first step before examining deeper OSPF-specific settings.
What should I do if I get this 200-301 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 200-301 OSPF questions on adjacency and route selection.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 14, 2026
This 200-301 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 200-301 exam.
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