Question 1,114 of 2,015

OSPF →mediumMultiple ChoiceObjective-mapped

Quick Answer

The answer is that the forwarding address in the type 5 LSA is not reachable via an OSPF internal route. OSPF enforces a strict reachability check before installing external routes: the forwarding address (FA) field in a Type 5 LSA must be reachable through an intra-area or inter-area OSPF route. If the FA points to a prefix that is only reachable via another external route or is simply absent from the OSPF domain, the router will ignore the LSA entirely, keeping the route out of the routing table despite the LSA being present in the database and the neighbor adjacency being up. On the ENCOR 350-401 exam, this concept tests your understanding of OSPF external route installation rules, often appearing as a trick where candidates assume a full database means a full routing table. A common trap is forgetting that the FA is often set to 0.0.0.0 (meaning the originating ASBR is the next hop) but when non-zero, it must be an internal OSPF route. Memory tip: FA must be FA-miliar to OSPF internally, or the route stays out.

CCNP OSPF Practice Question

This 350-401 practice question tests your understanding of ospf. Read the scenario carefully and evaluate each option against the stated constraints before committing to an answer. 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 network uses OSPF as its IGP. The network engineer notices that a particular route learned via OSPF is not being installed in the routing table, even though the neighbor adjacency is up and the route appears in the OSPF database. The route is an external route redistributed from EIGRP. What is the most likely cause?

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.

Question 1mediummultiple choice

Review the full OSPF breakdown →

A
The OSPF process ID is different on the routers.
Why wrong: Incorrect because OSPF process ID is locally significant and does not affect route installation.
B
The external route has a higher administrative distance than the internal route.
Why wrong: Incorrect because administrative distance is used when comparing routes from different sources; the OSPF external route is not being installed at all, not being compared.
C
The forwarding address in the type 5 LSA is not reachable via an OSPF internal route.
Correct because OSPF requires the forwarding address to be reachable via an intra-area or inter-area route; otherwise, the external route is not installed.
D
The OSPF metric for the external route is too high.
Why wrong: Incorrect because OSPF does not have a maximum metric that prevents route installation; even high metrics are installed.

Full breakdown with real-world context →

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 forwarding address in the type 5 LSA is not reachable via an OSPF internal route.

Option C is correct because OSPF requires the forwarding address (FA) in a Type 5 LSA to be reachable via an OSPF internal route (intra-area or inter-area) for the external route to be installed in the routing table. If the FA is not reachable, the router will ignore the LSA and not install the route, even though the LSA exists in the OSPF database and the neighbor adjacency is up.

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 OSPF process ID is different on the routers.
Why it's wrong here
Incorrect because OSPF process ID is locally significant and does not affect route installation.
✗
The external route has a higher administrative distance than the internal route.
Why it's wrong here
Incorrect because administrative distance is used when comparing routes from different sources; the OSPF external route is not being installed at all, not being compared.
✓
The forwarding address in the type 5 LSA is not reachable via an OSPF internal route.
Why this is correct
Correct because OSPF requires the forwarding address to be reachable via an intra-area or inter-area route; otherwise, the external route is not installed.
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 OSPF metric for the external route is too high.
Why it's wrong here
Incorrect because OSPF does not have a maximum metric that prevents route installation; even high metrics are installed.

Common exam traps

Common exam trap: answer the scenario, not the keyword

Cisco often tests the forwarding address reachability requirement for Type 5 LSAs, and the trap here is that candidates assume any route in the OSPF database will automatically be installed, ignoring the recursive lookup condition for external routes with a non-zero forwarding address.

Detailed technical explanation

How to think about this question

The forwarding address field in a Type 5 LSA is set to 0.0.0.0 by default when the ASBR is the next hop, but if the redistributing router has a non-zero forwarding address (e.g., from a directly connected next hop), OSPF performs a recursive lookup to ensure that address is reachable via an OSPF internal route. If the forwarding address is not in the OSPF routing table, the route is not installed, which can happen in multi-homed redistribution scenarios or when using MPLS/VPN. This behavior is defined in RFC 2328, Section 16.4.

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.

What to study next

Got this wrong? Here's your next step.

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

Related 350-401 practice-question pages

Use these pages to review the topic behind this question. This is how one missed question becomes focused revision.

Architecture practice questions

Practise 350-401 questions linked to Architecture.

Enterprise Network Design practice questions

Practise 350-401 questions linked to Enterprise Network Design.

SD-Access Architecture practice questions

Practise 350-401 questions linked to SD-Access Architecture.

SD-WAN Architecture practice questions

Practise 350-401 questions linked to SD-WAN Architecture.

QoS Architecture practice questions

Practise 350-401 questions linked to QoS Architecture.

Virtualization practice questions

Practise 350-401 questions linked to Virtualization.

Network Function Virtualization practice questions

Practise 350-401 questions linked to Network Function Virtualization.

Virtual Machines and Hypervisors practice questions

Practise 350-401 questions linked to Virtual Machines and Hypervisors.

VRF and Path Isolation practice questions

Practise 350-401 questions linked to VRF and Path Isolation.

Infrastructure practice questions

Practise 350-401 questions linked to Infrastructure.

OSPF practice questions

Practise 350-401 questions linked to OSPF.

BGP practice questions

Practise 350-401 questions linked to BGP.

Practice this exam

Start a free 350-401 practice session

Short sessions build daily habit. Longer sessions build exam-day stamina. Try a timed session to simulate real conditions.

10 questions 20 questions 30 questions 50 questions Timed 30

350-401 practice-test guide →Study guide →Browse all practice tests

FAQ

Questions learners often ask

What does this 350-401 question test?

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

What is the correct answer to this question?

The correct answer is: The forwarding address in the type 5 LSA is not reachable via an OSPF internal route. — Option C is correct because OSPF requires the forwarding address (FA) in a Type 5 LSA to be reachable via an OSPF internal route (intra-area or inter-area) for the external route to be installed in the routing table. If the FA is not reachable, the router will ignore the LSA and not install the route, even though the LSA exists in the OSPF database and the neighbor adjacency is up.

What should I do if I get this 350-401 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.

About these practice questions

Courseiva creates original exam-style practice questions with explanations and wrong-answer analysis. It does not publish real exam questions, exam dumps, or protected exam content. Learn why practice questions differ from exam dumps →

How Courseiva writes practice questions · Editorial policy

Same concept, more angles

1 more ways this is tested on 350-401

These questions test the same concept from different angles. Work through them to make sure you can recognise it however the exam phrases it.

Variation 1. A network engineer is troubleshooting an OSPF issue where a router is not learning a route to a network that is advertised via a type 5 LSA from an ASBR. The engineer checks the OSPF database and sees the type 5 LSA, but the route is not in the routing table. The forwarding address in the LSA is 0.0.0.0. What is the most likely cause?

hard

✓ A.The ASBR is not reachable via an OSPF internal route.
B.The type 5 LSA has a metric of 16777215.
C.The OSPF process ID on the ASBR is different from the other routers.
D.The type 5 LSA is being filtered by an outbound route filter.

Why A: When a Type 5 LSA has a forwarding address of 0.0.0.0, OSPF routers will use the ASBR as the next hop for the external route. For the route to be installed in the routing table, the ASBR must be reachable via an OSPF intra-area or inter-area route. If the ASBR is not reachable (e.g., no valid OSPF route to the ASBR's router ID), the Type 5 LSA is considered unreachable and is not installed, even though it exists in the OSPF database.

Keep practising

Question Discussion

Share a tip, memory trick, or ask about the reasoning behind this question. Do not post real exam questions, leaked content, braindumps, or copyrighted exam material. Comments are moderated and may be removed without notice.

Loading comments…

This 350-401 practice question is part of Courseiva's free Cisco certification practice question bank. Courseiva provides original exam-style practice questions with explanations, topic-based practice, mock exams, readiness tracking, and study analytics to help learners prepare for the 350-401 exam.