The answer is that the BGP routes are not installed because their next-hop addresses are unreachable, despite the presence of a default route. In BGP, a route is only placed into the IP routing table if the router has a valid, non-default route to the next-hop IP address; a default route does not satisfy this requirement because BGP treats it as a route of last resort, not a specific path to the next-hop. On the ENCOR 350-401 exam, this scenario tests your understanding of the BGP next-hop reachability check, a common trap where candidates assume a default route will suffice for BGP route installation. The key distinction is that BGP requires an exact match in the routing table for the next-hop, not a default. A helpful memory tip is: “BGP needs a specific route to the next-hop, not a default umbrella.”
CCNP Infrastructure Practice Question
This 350-401 practice question tests your understanding of infrastructure. 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 multinational organization has a BGP-based MPLS VPN network. The CE router at a branch office is connected to two PE routers (PE1 and PE2) in the service provider network. The branch uses eBGP to exchange routes with the PEs. The network administrator notices that the branch can reach some destinations but not others. The BGP table on the CE shows routes with next-hop set to the PE loopback addresses, but those loopbacks are not reachable. The CE has a default route pointing to the PEs. What is the most likely cause of the issue?
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 addresses of the BGP routes are not reachable.
The CE router learns BGP routes from the PE routers with next-hop addresses set to the PE loopback interfaces. For these routes to be installed in the routing table, the CE must have a route to the next-hop IP address. Since the CE only has a default route pointing to the PEs and the PE loopbacks are not directly connected or reachable via any specific route, the BGP routes remain hidden (not installed) because the next-hop is unreachable. This is the most likely cause of partial reachability.
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 next-hop addresses of the BGP routes are not reachable.
Why this is correct
Unreachable next-hop prevents route installation.
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 default route on the CE is overriding the BGP routes.
AS path length does not prevent route installation.
✗
The CE is not advertising its routes to the PEs.
Why it's wrong here
The issue is with receiving routes, not advertising.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Cisco often tests the BGP next-hop reachability rule, where candidates mistakenly think a default route satisfies the next-hop check, but BGP requires a specific route to the next-hop address (not a default route) for the route to be installed in the routing table.
Detailed technical explanation
How to think about this question
In BGP, the next-hop reachability check is a fundamental requirement for route installation. For eBGP learned routes, the CE must have an IGP route (or static route) to the next-hop IP address, which in MPLS VPN is typically the PE loopback. If the CE only has a default route, the BGP next-hop is not considered reachable because BGP does not resolve next-hops via default routes by default (unless 'next-hop-unchanged' or 'disable-next-hop-check' is configured). In real-world scenarios, this is often resolved by configuring a static route on the CE pointing to the PE loopback or using 'next-hop-self' on the PE to set the next-hop to the PE's directly connected interface.
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 small business has 20 workstations on the 192.168.1.0/24 network and one public IP from its ISP. The router uses PAT (NAT overload) so all 20 devices share one public address using different source ports. NAT questions test whether you understand the four address terms and which direction each translation applies.
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.
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
What is the correct answer to this question?
The correct answer is: The next-hop addresses of the BGP routes are not reachable. — The CE router learns BGP routes from the PE routers with next-hop addresses set to the PE loopback interfaces. For these routes to be installed in the routing table, the CE must have a route to the next-hop IP address. Since the CE only has a default route pointing to the PEs and the PE loopbacks are not directly connected or reachable via any specific route, the BGP routes remain hidden (not installed) because the next-hop is unreachable. This is the most likely cause of partial reachability.
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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