The root cause is that the BGP route 172.16.0.0/24 is not installed in the routing table, typically due to next-hop unreachability. When redistributing BGP into OSPF, the OSPF process can only advertise prefixes that exist in the global routing table; if the BGP next-hop is not reachable via an IGP or static route, the route remains in the BGP table as invalid and is never injected into OSPF. This scenario tests your understanding of the redistribution dependency on route validity, a common pitfall in the Cisco CCNP ENARSI 300-410 exam. The giveaway is the OSPF Type-5 LSA showing a metric of 16777215, which is the maximum value OSPF uses to signal an unreachable route—often a trap where candidates overlook the BGP next-hop check. Remember the memory tip: "No next-hop, no OSPF drop"—always verify BGP table validity before troubleshooting redistribution filters.
300-410 OSPF Troubleshooting (v2/v3) Practice Question
This 300-410 practice question tests your understanding of ospf troubleshooting (v2/v3). 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 router R1 is redistributing BGP routes into OSPF, but some prefixes are missing. Router R1 has the following relevant configuration: router bgp 65000 neighbor 10.1.1.2 remote-as 65001 address-family ipv4 unicast neighbor 10.1.1.2 activate network 172.16.0.0 mask 255.255.255.0 router ospf 1 redistribute bgp 65000 subnets metric 100 Router R2 shows: show ip ospf database external 172.16.0.0 OSPF Router with ID 10.1.1.1 Type-5 AS External Link States LS age: 600 Options: (No TOS-capability, DC) LS Type: AS External Link Link State ID: 172.16.0.0 Advertising Router: 10.1.1.1 LS Seq Number: 80000001 Checksum: 0x1234 Length: 36 Network Mask: /24 Metric Type: 2 (Larger than any valid route) TOS: 0 Metric: 16777215 Forward Address: 0.0.0.0 External Route Tag: 0 What is the root cause?
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The BGP route 172.16.0.0/24 is not in the routing table; check BGP table and next-hop reachability.
The metric of 16777215 (max) indicates the route is unreachable. This happens when BGP route is not valid (e.g., missing next-hop reachability) or when route-map sets metric to max. The BGP route may not be installed in routing table due to missing network statement or next-hop issue. The correct fix is to ensure BGP route is valid and reachable.
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 BGP route 172.16.0.0/24 is not in the routing table; check BGP table and next-hop reachability.
Why this is correct
OSPF redistributes only routes from routing table; if BGP route is not installed, OSPF may inject a default or max-metric route.
The metric 100 is too low; increase to match OSPF cost.
Why it's wrong here
Metric value does not cause missing routes; the max metric indicates a problem.
✗
The subnets keyword is missing; add it to redistribute classful routes.
Why it's wrong here
Subnets is present; missing it would not cause max metric.
✗
The OSPF process ID is mismatched; use process 1 on both routers.
Why it's wrong here
Process ID is local; mismatch does not affect redistribution.
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.
OSPF Troubleshooting (v2/v3) — This question tests OSPF Troubleshooting (v2/v3) — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The BGP route 172.16.0.0/24 is not in the routing table; check BGP table and next-hop reachability. — The metric of 16777215 (max) indicates the route is unreachable. This happens when BGP route is not valid (e.g., missing next-hop reachability) or when route-map sets metric to max. The BGP route may not be installed in routing table due to missing network statement or next-hop issue. The correct fix is to ensure BGP route is valid and reachable.
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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