Question 450 of 1,020

Network Protocols →hardMultiple ChoiceObjective-mapped

OSPF Multicast Forwarding Issue for CompTIA A+ 220-1201

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.

A
The routers are using different OSPF area IDs.
Why wrong: Different area IDs can cause issues, but OSPF can still exchange routes between areas via ABRs; the problem is more likely multicast.
B
The routers are using different OSPF process IDs.
Why wrong: OSPF process IDs are local to each router and do not affect neighbor formation.
C
Multicast is not being forwarded between the routers.
OSPF relies on multicast for neighbor discovery and route exchange; blocking multicast prevents this.
D
The routers have mismatched subnet masks on their interfaces.
Why wrong: Mismatched subnet masks would prevent direct pings, which are working, so this is not the issue.

Full breakdown with real-world context →

Quick Answer

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.

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

✓

Multicast is not being forwarded between the routers.

OSPF uses multicast address 224.0.0.5 (AllSPFRouters) to exchange hello packets and LSAs. If multicast forwarding is disabled or blocked between the routers (e.g., by an ACL, switchport configuration, or lack of multicast routing), OSPF neighbors will not form, and routes will not be exchanged. Since both offices can ping each other's directly connected interfaces, Layer 3 unicast connectivity exists, but OSPF's multicast-based neighbor discovery is failing.

Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.
Related concept
Read the scenario before looking for a memorised 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: answer the scenario, not the keyword

A common mistake is to think that OSPF process IDs must match between routers, but the trap here is that candidates overlook the multicast dependency and instead focus on area IDs or subnet masks, even though the ping success already eliminates those Layer 3 issues.

Detailed technical explanation

How to think about this question

OSPF relies on IP protocol 89 and uses multicast addresses 224.0.0.5 (for all OSPF routers) and 224.0.0.6 (for DR/BDR). On multi-access networks, if IGMP snooping or switchport filters block these multicast frames, OSPF hellos never reach the neighbor. Additionally, on point-to-point links, OSPF can be configured to use unicast (neighbor command), but by default it uses multicast; the failure here indicates multicast is not being forwarded, which is a common issue when OSPF is run over a VLAN trunk or through a firewall that drops multicast traffic.

KKey Concepts to Remember

Read the scenario before looking for a memorised answer.
Find the constraint that changes the correct option.
Eliminate answers that are true in general but not in this case.

TExam Day Tips

Watch for words such as best, first, most likely and least administrative effort.
Review why wrong options are wrong, not only why the correct option is correct.

Key takeaway

Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.

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.

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.

What to study next

Got this wrong? Here's your next step.

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FAQ

Questions learners often ask

What does this 220-1201 question test?

Network Protocols — This question tests Network Protocols — Read the scenario before looking for a memorised answer..

What is the correct answer to this question?

The correct answer is: Multicast is not being forwarded between the routers. — OSPF uses multicast address 224.0.0.5 (AllSPFRouters) to exchange hello packets and LSAs. If multicast forwarding is disabled or blocked between the routers (e.g., by an ACL, switchport configuration, or lack of multicast routing), OSPF neighbors will not form, and routes will not be exchanged. Since both offices can ping each other's directly connected interfaces, Layer 3 unicast connectivity exists, but OSPF's multicast-based neighbor discovery is failing.

What should I do if I get this 220-1201 question wrong?

Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.

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?

Read the scenario before looking for a memorised answer.

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