- A
The redistributed route's next-hop is not reachable via OSPF, causing the forwarding address to be 0.0.0.0, but R1's router ID is not reachable from R3 due to a missing OSPF network statement.
If the next-hop is not in OSPF, the forwarding address is set to 0.0.0.0, meaning the packet should be sent to the advertising router (R1). If R1's router ID is not reachable (e.g., R1's OSPF interfaces are not advertised correctly), R3 cannot reach the route.
- B
The route-map RMAP-OSPF is setting the metric to 20, which is too high and causes the route to be suppressed.
Why wrong: The metric 20 is the default for E2 routes and does not suppress the route.
- C
The EIGRP process on R1 has a distribute-list blocking the route.
Why wrong: The route is being redistributed (shown in OSPF database), so EIGRP is not blocking it.
- D
The OSPF process on R1 has a route-map applied inbound that filters the route.
Why wrong: The route is in the OSPF database, so it is not filtered inbound.
Quick Answer
The answer is that the redistributed route's next-hop is not reachable via OSPF, which forces the forwarding address to 0.0.0.0, but R1’s router ID is unreachable from R3 due to a missing OSPF network statement. When OSPF redistributes an external route and the original next-hop is not within the OSPF domain, the router sets the forwarding address to 0.0.0.0, meaning other OSPF routers must use the originating router (R1) as the next-hop. If R1’s router ID is not reachable—often because its OSPF process lacks a network statement covering the interface used for router ID reachability—the route is installed but unreachable. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of OSPF external route propagation and the forwarding address field, a common trap where candidates overlook the requirement that the originating router’s OSPF router ID must be reachable. A key memory tip: if the forwarding address is 0.0.0.0, the router ID must be in the OSPF routing table—otherwise, the route is a dead end.
300-410 Route Maps and Route Filtering Practice Question
This 300-410 practice question tests your understanding of route maps and route filtering. 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 enterprise is redistributing EIGRP into OSPF on router R1. R1 has the following configuration: router eigrp 100 redistribute ospf 1 metric 10000 100 255 1 1500 route-map RMAP-OSPF. Router R2 (OSPF-only) shows: 'show ip route 10.1.1.0' as an O E2 route with metric 20, but 'show ip ospf database external 10.1.1.0' shows the forwarding address is 0.0.0.0. Router R3 (also OSPF) cannot reach 10.1.1.0. What is the root cause?
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
The redistributed route's next-hop is not reachable via OSPF, causing the forwarding address to be 0.0.0.0, but R1's router ID is not reachable from R3 due to a missing OSPF network statement.
When redistributing into OSPF, if the redistributed route's next-hop is not reachable via OSPF (e.g., it is an EIGRP-learned route with a next-hop that is not in the OSPF domain), the forwarding address is set to 0.0.0.0. This causes other OSPF routers to try to reach the originating router (R1) as the next-hop. However, if R1's OSPF router ID is not reachable or if there is a filtering issue, the route may be installed but unreachable. In this scenario, the route-map RMAP-OSPF may be filtering the route or setting incorrect metrics, but the forwarding address issue suggests that the next-hop is not being advertised correctly. The root cause is likely that the redistributed route's next-hop is not in the OSPF domain, so R1 should use a route-map to set the forwarding address to itself (e.g., set forwarding-address 0.0.0.0 is default, but if the next-hop is reachable, it should be set to the router's own interface IP).
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 redistributed route's next-hop is not reachable via OSPF, causing the forwarding address to be 0.0.0.0, but R1's router ID is not reachable from R3 due to a missing OSPF network statement.
Why this is correct
If the next-hop is not in OSPF, the forwarding address is set to 0.0.0.0, meaning the packet should be sent to the advertising router (R1). If R1's router ID is not reachable (e.g., R1's OSPF interfaces are not advertised correctly), R3 cannot reach the route.
Related concept
OSPF neighbours must agree on key parameters.
- ✗
The route-map RMAP-OSPF is setting the metric to 20, which is too high and causes the route to be suppressed.
Why it's wrong here
The metric 20 is the default for E2 routes and does not suppress the route.
- ✗
The EIGRP process on R1 has a distribute-list blocking the route.
- ✗
The OSPF process on R1 has a route-map applied inbound that filters the route.
Why it's wrong here
The route is in the OSPF database, so it is not filtered inbound.
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.
Trap categories for this question
Command / output trap
The route is being redistributed (shown in OSPF database), so EIGRP is not blocking it.
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.
What to study next
Got this wrong? Here's your next step.
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.
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FAQ
Questions learners often ask
What does this 300-410 question test?
Route Maps and Route Filtering — This question tests Route Maps and Route Filtering — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The redistributed route's next-hop is not reachable via OSPF, causing the forwarding address to be 0.0.0.0, but R1's router ID is not reachable from R3 due to a missing OSPF network statement. — When redistributing into OSPF, if the redistributed route's next-hop is not reachable via OSPF (e.g., it is an EIGRP-learned route with a next-hop that is not in the OSPF domain), the forwarding address is set to 0.0.0.0. This causes other OSPF routers to try to reach the originating router (R1) as the next-hop. However, if R1's OSPF router ID is not reachable or if there is a filtering issue, the route may be installed but unreachable. In this scenario, the route-map RMAP-OSPF may be filtering the route or setting incorrect metrics, but the forwarding address issue suggests that the next-hop is not being advertised correctly. The root cause is likely that the redistributed route's next-hop is not in the OSPF domain, so R1 should use a route-map to set the forwarding address to itself (e.g., set forwarding-address 0.0.0.0 is default, but if the next-hop is reachable, it should be set to the router's own interface IP).
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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Last reviewed: Jun 18, 2026
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