Quick Answer
The answer is that the next-hop IP address is not reachable. When a BGP session is established but no routes are received, the TCP connection and BGP open messages have succeeded, but the router cannot install the advertised prefixes because the next-hop address—typically the eBGP neighbor’s interface IP—is missing from the local routing table. This is a classic BGP route advertisement rule: a router will not accept or propagate a route unless the next-hop is reachable via an IGP or static route. On the ENCOR 350-401 exam, this scenario tests your understanding that a successful ping to the neighbor does not guarantee the next-hop is in the routing table; the trap is confusing Layer 3 reachability with BGP’s next-hop validation logic. A reliable memory tip is “BGP won’t trust what it can’t reach”—if the next-hop isn’t in the RIB, the route stays hidden.
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.
An engineer is troubleshooting a BGP routing issue. The router is not receiving routes from an eBGP neighbor. The neighbor is reachable via ping. The BGP session is established and in the Established state. 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.
The AS number is incorrect.
Why wrong: If AS number is incorrect, session would not establish.
The BGP session is using an incorrect interface.
Why wrong: Session established means interface is fine.
The network is not advertised under BGP.
Why wrong: This would affect route advertisement, not receipt.
The next-hop IP address is not reachable.
eBGP routes require reachable next-hop to be installed.
Answer choices
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.
The BGP session is established, which confirms that the TCP connection and BGP open messages have been successfully exchanged. However, routes are not being received. The most common cause in this scenario is that the next-hop IP address for the advertised routes is not reachable via the router's routing table. eBGP by default sets the next-hop to the IP address of the neighbor's interface, and if that address is not reachable (e.g., missing route, incorrect static route, or interface down), the router will not install the routes into the BGP table or the routing table.
Key principle: Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Answer analysis
For each option: why learners choose it and why it is or isn't the right answer here.
The AS number is incorrect.
Why it's wrong here
If AS number is incorrect, session would not establish.
The BGP session is using an incorrect interface.
Why it's wrong here
Session established means interface is fine.
The network is not advertised under BGP.
Why it's wrong here
This would affect route advertisement, not receipt.
The next-hop IP address is not reachable.
Why this is correct
eBGP routes require reachable next-hop to be 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.
Common exam traps
Cisco often tests the distinction between a BGP session being Established (Layer 4 connectivity) and the actual route installation (Layer 3 reachability), leading candidates to overlook the next-hop reachability requirement.
Detailed technical explanation
BGP uses the next-hop attribute to determine the path to a prefix. For eBGP, the next-hop is typically the IP address of the neighbor's interface that is directly connected. If the router does not have a route to that next-hop (e.g., no directly connected route, or a missing static route), the BGP best-path selection process will mark the route as invalid and not install it in the routing table. This is a common issue in multi-hop eBGP scenarios or when using loopback interfaces without proper reachability. The 'show ip bgp' command will display the route with a 'r' (RIB-failure) or 'd' (not installed) flag.
Key takeaway
Answer the scenario, not the keyword: identify the specific constraint before choosing the most familiar-sounding option.
Real-world example
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
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FAQ
Infrastructure — This question tests Infrastructure — Read the scenario before looking for a memorised answer..
The correct answer is: The next-hop IP address is not reachable. — The BGP session is established, which confirms that the TCP connection and BGP open messages have been successfully exchanged. However, routes are not being received. The most common cause in this scenario is that the next-hop IP address for the advertised routes is not reachable via the router's routing table. eBGP by default sets the next-hop to the IP address of the neighbor's interface, and if that address is not reachable (e.g., missing route, incorrect static route, or interface down), the router will not install the routes into the BGP table or the routing table.
Identify which exam domain this question belongs to, review the core concept, then practise similar questions from the same domain.
Yes — watch for: "most likely". Probability qualifier — the question wants the most probable cause or outcome, not a guaranteed one. Eliminate low-probability options.
Read the scenario before looking for a memorised answer.
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Last reviewed: Jun 11, 2026
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