The answer is that the BGP routes are not valid because the next-hop IP address is unreachable. For a BGP route to be considered valid and installed in the IP routing table, the router must have a reachable path to the next-hop address, typically learned via an IGP like OSPF or EIGRP, or through a static route. If that next hop is missing from the routing table, the BGP process marks the route as not valid—often shown with an 'r' flag in the 'show ip bgp' output—even though the route was successfully received from the eBGP neighbor. On the ENCOR 350-401 exam, this concept tests your understanding of BGP path selection and the fundamental prerequisite that next-hop reachability must be resolved before any BGP route can be used. A common trap is assuming the route is invalid due to a prefix-list or AS-path issue, but the most frequent cause is simply an unreachable next hop. Memory tip: “No route to the hop, the BGP route will drop.”
CCNP Network Assurance Practice Question
This 350-401 practice question tests your understanding of network assurance. 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 network administrator is troubleshooting a BGP routing issue where routes from an eBGP neighbor are not being installed in the routing table. The 'show ip bgp' output shows the routes are received but not valid. 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.
Answer the question above first, then reveal the full breakdown to understand why each option is right or wrong.
Correct answer & explanation
✓
The next-hop IP address is not reachable.
For a BGP route to be considered valid and installed in the routing table, the next-hop IP address must be reachable via an IGP or a static route. If the next hop is not reachable, the route will appear in the 'show ip bgp' output but will be marked as not valid (often with a 'r' for received but not valid). This is the most common cause when routes are received from an eBGP neighbor but not installed.
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 AS-path contains the local AS number.
Why it's wrong here
Option C is wrong because that would cause a loop prevention error, but the route would be marked as invalid with a different reason.
✓
The next-hop IP address is not reachable.
Why this is correct
Option B is correct because if the next-hop is not reachable, the 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.
✗
BGP synchronization is enabled.
Why it's wrong here
Option A is wrong because synchronization is disabled by default and rarely used.
✗
The maximum-prefix limit has been exceeded.
Why it's wrong here
Option D is wrong because that would tear down the BGP session.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the distinction between routes being received in the BGP table versus being installed in the routing table, and the trap here is that candidates confuse synchronization (a deprecated feature) with the next-hop reachability requirement, which is the immediate cause of the 'not valid' status.
Detailed technical explanation
How to think about this question
BGP validates the next hop by performing a recursive lookup in the routing table. If the next-hop address is not reachable (e.g., missing a route to the directly connected interface or a static route), the route remains in the BGP table with a status of 'r' (received) but not valid. This behavior is defined in RFC 4271 and is independent of the BGP synchronization rule, which only applies to iBGP routes in older IOS versions.
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.
Network Assurance — This question tests Network Assurance — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The next-hop IP address is not reachable. — For a BGP route to be considered valid and installed in the routing table, the next-hop IP address must be reachable via an IGP or a static route. If the next hop is not reachable, the route will appear in the 'show ip bgp' output but will be marked as not valid (often with a 'r' for received but not valid). This is the most common cause when routes are received from an eBGP neighbor but not installed.
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.
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Question Discussion
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