- A
BGP synchronization is enabled, and the route is not present in the IGP routing table, so BGP does not advertise it to iBGP peers.
Why wrong: BGP synchronization only applies to routes learned from IBGP being advertised to EBGP; it does not affect advertising EBGP routes to IBGP neighbors. Also, synchronization is disabled by default in modern IOS.
- B
The iBGP session is using a loopback interface that is not reachable via the IGP.
Why wrong: The scenario explicitly states the next-hop is reachable via IGP, so this is not the cause.
- C
The BGP table shows the route as 'valid' but 'not best' due to a higher local preference.
Correct. If the route is valid but not the best path (e.g., another IBGP route has a higher local preference), BGP will not advertise it to any neighbor, including IBGP peers.
- D
The router is configured with 'bgp bestpath med missing-as-worst', which affects MED comparison but not advertisement.
Why wrong: The 'bgp bestpath med missing-as-worst' command affects MED comparison only and does not prevent route advertisement.
BGP iBGP Route Advertisement Troubleshooting
This 300-410 practice question tests your understanding of netflow and flexible netflow. The scenario asks you to isolate a root cause — eliminate options that address a different problem before choosing. A key principle to apply: bGP Best Path Selection. 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 configures BGP between two routers in the same AS. The iBGP session is established, but the routes learned from eBGP are not being advertised to the iBGP neighbor. The engineer verifies that the next-hop is reachable via IGP. Which is the most likely explanation?
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.
Quick Answer
The answer is BGP synchronization being enabled, with the route absent from the IGP routing table, preventing advertisement to the iBGP neighbor. This occurs because when synchronization is active, BGP will not advertise a route learned from an eBGP peer to an iBGP neighbor unless that same route is also present in the IGP routing table, ensuring the transit AS is fully reachable. On the Cisco CCNP ENARSI 300-410 exam, this tests your understanding of the BGP route advertisement to iBGP neighbor synchronization rule, a classic pitfall where engineers verify next-hop reachability but overlook the synchronization requirement. A common trap is assuming that a reachable next-hop alone guarantees advertisement, but synchronization adds an extra IGP presence check. Remember: if synchronization is on, the route must be in both BGP and IGP tables before it is shared with iBGP peers. Memory tip: “Sync before you share” — the IGP must have the route before BGP will advertise it internally.
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 BGP table shows the route as 'valid' but 'not best' due to a higher local preference.
In BGP, only the best path to a destination is advertised to neighbors. The engineer confirmed that the next-hop is reachable via IGP, so reachability is not the issue. If the eBGP-learned route is not selected as the best path (e.g., due to a higher local preference from another IBGP path), it will not be advertised to IBGP peers. This explains why the route is not being advertised despite the session being established and next-hop reachable.
Key principle: BGP Best Path Selection
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✗
BGP synchronization is enabled, and the route is not present in the IGP routing table, so BGP does not advertise it to iBGP peers.
Why it's wrong here
BGP synchronization only applies to routes learned from IBGP being advertised to EBGP; it does not affect advertising EBGP routes to IBGP neighbors. Also, synchronization is disabled by default in modern IOS.
- ✗
The iBGP session is using a loopback interface that is not reachable via the IGP.
Why it's wrong here
The scenario explicitly states the next-hop is reachable via IGP, so this is not the cause.
- ✓
The BGP table shows the route as 'valid' but 'not best' due to a higher local preference.
Why this is correct
Correct. If the route is valid but not the best path (e.g., another IBGP route has a higher local preference), BGP will not advertise it to any neighbor, including IBGP peers.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
BGP Best Path Selection
- ✗
The router is configured with 'bgp bestpath med missing-as-worst', which affects MED comparison but not advertisement.
Why it's wrong here
The 'bgp bestpath med missing-as-worst' command affects MED comparison only and does not prevent route advertisement.
Common exam traps
Common exam trap: answer the scenario, not the keyword
Many certification questions include familiar terms but test a specific constraint. Read the exact wording before choosing an answer that is generally true but wrong for this case.
Trap categories for this question
Command / output trap
The 'bgp bestpath med missing-as-worst' command affects MED comparison only and does not prevent route advertisement.
Scenario analysis trap
The scenario explicitly states the next-hop is reachable via IGP, so this is not the cause.
Detailed technical explanation
How to think about this question
Treat this as a scenario question. Identify the problem, the constraint, and the best action. Then compare each option against those facts.
KKey Concepts to Remember
- BGP Best Path Selection
- Local Preference
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
BGP Best Path Selection
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 bGP Best Path Selection, then practise related 300-410 questions on the same topic to reinforce the concept.
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NetFlow and Flexible NetFlow — study guide chapter
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NetFlow and Flexible NetFlow practice questions
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FAQ
Questions learners often ask
What does this 300-410 question test?
NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — BGP Best Path Selection.
What is the correct answer to this question?
The correct answer is: The BGP table shows the route as 'valid' but 'not best' due to a higher local preference. — In BGP, only the best path to a destination is advertised to neighbors. The engineer confirmed that the next-hop is reachable via IGP, so reachability is not the issue. If the eBGP-learned route is not selected as the best path (e.g., due to a higher local preference from another IBGP path), it will not be advertised to IBGP peers. This explains why the route is not being advertised despite the session being established and next-hop reachable.
What should I do if I get this 300-410 question wrong?
Review bGP Best Path Selection, then practise related 300-410 questions on the same topic to reinforce the concept.
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?
BGP Best Path Selection
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Last reviewed: Jun 19, 2026
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