The answer is that multicast is not being forwarded between the routers, which is the most likely protocol issue preventing OSPF route exchange. OSPF relies on the multicast addresses 224.0.0.5 (for all OSPF routers) and 224.0.0.6 (for designated routers) to discover neighbors and share routing updates. Even though unicast connectivity works—as shown by the successful pings between directly connected interfaces—multicast traffic can be blocked by access control lists, switch port settings, or a lack of multicast routing support, causing OSPF neighbors to never form. On the CompTIA A+ Core 1 220-1201 exam, this scenario tests your understanding that OSPF is a link-state protocol dependent on multicast, not just unicast reachability. A common trap is assuming that if pings work, all routing protocols will function; the key distinction is that OSPF specifically requires multicast forwarding. Memory tip: OSPF talks to “all routers” on 224.0.0.5, so if that multicast “party” is blocked, no routes get exchanged.
220-1101 Network Protocols Practice Question
This 220-1201 practice question tests your understanding of network protocols. 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 administrator configures a router to use OSPF for dynamic routing between two branch offices. After configuration, routes from one office are not appearing in the other's routing table. Both offices can ping each other's directly connected interfaces. Which protocol issue is most likely preventing route exchange?
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
✓
Multicast is not being forwarded between the routers.
OSPF uses multicast addresses (224.0.0.5 and 224.0.0.6) to exchange routing information. If the network does not support multicast or if multicast is blocked by access lists or switch configurations, OSPF neighbors cannot form and routes are not exchanged. Since direct pings work, unicast connectivity is fine, but multicast is required for OSPF.
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.
✗
The routers are using different OSPF area IDs.
Why it's wrong here
Different area IDs can cause issues, but OSPF can still exchange routes between areas via ABRs; the problem is more likely multicast.
✗
The routers are using different OSPF process IDs.
Why it's wrong here
OSPF process IDs are local to each router and do not affect neighbor formation.
✓
Multicast is not being forwarded between the routers.
Why this is correct
OSPF relies on multicast for neighbor discovery and route exchange; blocking multicast prevents this.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The routers have mismatched subnet masks on their interfaces.
Why it's wrong here
Mismatched subnet masks would prevent direct pings, which are working, so this is not the issue.
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 220-1201 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 220-1201 OSPF questions on adjacency and route selection.
Network Protocols — This question tests Network Protocols — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: Multicast is not being forwarded between the routers. — OSPF uses multicast addresses (224.0.0.5 and 224.0.0.6) to exchange routing information. If the network does not support multicast or if multicast is blocked by access lists or switch configurations, OSPF neighbors cannot form and routes are not exchanged. Since direct pings work, unicast connectivity is fine, but multicast is required for OSPF.
What should I do if I get this 220-1201 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 220-1201 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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