The correct answer is that R3 prefers the route via R1 because OSPF selects external routes based on the lowest metric first, and the route from R1 has a metric of 20 compared to R2’s metric of 30. For OSPF type-5 external routes, the metric (cost) is the primary tiebreaker when the administrative distance is equal—both routes here inherit the default OSPF external AD of 110, so the lower metric wins. This question tests your understanding of the OSPF external route metric preference hierarchy, a common trap on the Cisco CCNP ENARSI 300-410 exam where candidates mistakenly think AS_PATH length or BGP attributes influence OSPF selection. Remember: OSPF ignores BGP path attributes entirely; only the redistributed metric matters unless you modify the OSPF external distance. Memory tip: "OSPF external—metric first, distance last."
300-410 Administrative Distance Practice Question
This 300-410 practice question tests your understanding of administrative distance. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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.
Router R1 and R2 are iBGP peers in the same AS. R1 learns a route 172.16.1.0/24 from an eBGP peer with AS_PATH 100 200. R2 learns the same prefix from another eBGP peer with AS_PATH 100. Both routers redistribute the route into OSPF with default administrative distance. R3, an OSPF internal router, sees two OSPF external routes for 172.16.1.0/24: one from R1 (type-5, metric 20) and one from R2 (type-5, metric 30). R3's 'show ip route 172.16.1.0' shows the route via R1. What is the root cause of R3 preferring the route via R1?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
R3 prefers the route with the lower metric (20) over the higher metric (30), which is correct OSPF behavior.
OSPF prefers external routes based on metric (cost) first for type-5 routes. The route from R1 has metric 20, R2 has metric 30, so R1 is preferred. However, if the administrative distance of OSPF external routes is changed via 'distance ospf external 150', then the route with lower AD is chosen. The correct answer is that the default AD for OSPF external is 110, so metric decides. The question implies a trick: the route from R2 might have a higher metric but lower AD due to a configuration, but the default is unchanged.
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.
✓
R3 prefers the route with the lower metric (20) over the higher metric (30), which is correct OSPF behavior.
Why this is correct
OSPF external routes are compared by metric first when the same type; lower metric wins.
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: R3 prefers the route with the lower metric (20) over the higher metric (30), which is correct OSPF behavior. — OSPF prefers external routes based on metric (cost) first for type-5 routes. The route from R1 has metric 20, R2 has metric 30, so R1 is preferred. However, if the administrative distance of OSPF external routes is changed via 'distance ospf external 150', then the route with lower AD is chosen. The correct answer is that the default AD for OSPF external is 110, so metric decides. The question implies a trick: the route from R2 might have a higher metric but lower AD due to a configuration, but the default is unchanged.
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.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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