The answer is that the EIGRP route is a summary route pointing to a null0 interface. This is correct because while EIGRP internal routes have an administrative distance of 90, which is lower than OSPF’s 110, the router will prefer the EIGRP route regardless of reachability. However, a null0 route is a local discard route created automatically by EIGRP when it summarizes, meaning any traffic matching that prefix is immediately dropped rather than forwarded. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of how administrative distance interacts with route types and next-hop behavior—a common trap is assuming a lower AD always guarantees successful packet delivery. The key insight is that administrative distance only determines which route is installed in the routing table, not whether the next-hop is valid. Remember the memory tip: “AD picks the winner, but null0 kills the packet.”
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. 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 is troubleshooting a routing loop between two routers. R1 and R2 are running both OSPF and EIGRP. R1 learns the prefix 172.16.1.0/24 via OSPF with AD 110 and via EIGRP internal with AD 90. The engineer notices that R1 installs the EIGRP route, but traffic to 172.16.1.0/24 is being dropped. What is the most likely issue?
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
✓
The EIGRP route is a summary route pointing to a null0 interface.
EIGRP internal routes have AD 90, which is lower than OSPF's 110, so EIGRP is preferred. However, if the EIGRP route points to a next-hop that is not reachable or is a backup path, traffic may be dropped. The root cause could be that the EIGRP route is learned via a secondary path that is not optimal or has a next-hop issue.
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 OSPF route has a better metric, but the EIGRP route is preferred due to lower AD.
Why it's wrong here
This is expected behavior; AD is compared first, so EIGRP is preferred. The issue is not about metric.
✓
The EIGRP route is a summary route pointing to a null0 interface.
Why this is correct
If R1 has an EIGRP summary route for 172.16.1.0/24 pointing to Null0, it will be installed with AD 90 and drop traffic, causing a black hole.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
The OSPF route has a higher AD because it is a type 5 LSA.
Why it's wrong here
OSPFAD is 110 for all routes; type 5 does not change AD.
✗
The EIGRP route is an external route with AD 170.
Why it's wrong here
The question states it is an internal route with AD 90, so this is not applicable.
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.
Administrative Distance — This question tests Administrative Distance — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The EIGRP route is a summary route pointing to a null0 interface. — EIGRP internal routes have AD 90, which is lower than OSPF's 110, so EIGRP is preferred. However, if the EIGRP route points to a next-hop that is not reachable or is a backup path, traffic may be dropped. The root cause could be that the EIGRP route is learned via a secondary path that is not optimal or has a next-hop issue.
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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