- A
AS path prepending only affects eBGP path selection; iBGP routers ignore AS path length when comparing paths from different eBGP routers.
iBGP uses the IGP metric to the next hop, not AS path length, so prepending does not influence iBGP decisions.
- B
The prepending was applied with 'set as-path prepend last-as 1', which only prepends the last AS, not the full path.
Why wrong: This would still affect eBGP, but iBGP still ignores AS path length.
- C
The iBGP neighbor has 'bgp bestpath as-path ignore' configured, which ignores AS path length entirely.
Why wrong: This would make all paths equal, but the issue is that prepending is not considered.
- D
The prepending was done on a route reflector, and the client does not see the prepended AS path.
Why wrong: Route reflectors pass the AS path unchanged, so the client would see it.
Quick Answer
The answer is that AS path prepending fails to influence iBGP neighbors because iBGP routers do not consider AS path length when selecting between routes learned from different eBGP routers. This is because AS path prepending artificially inflates the AS path attribute on eBGP updates, but once a route is learned via iBGP, the AS path length is not re-evaluated; iBGP relies on the IGP metric (or other attributes like MED and local preference) for best-path selection. On the Cisco CCNP ENARSI 300-410 exam, this concept tests your understanding of BGP path selection logic and the critical distinction between eBGP and iBGP behavior—a common trap is assuming prepending works universally. Remember the memory tip: "eBGP counts the hops, iBGP ignores the stops."
300-410 BGP Troubleshooting Practice Question
This 300-410 practice question tests your understanding of bgp troubleshooting. This is a configuration task: choose the command set that satisfies every stated requirement. Small differences — like 'secret' vs 'password' or 'transport input ssh' vs 'all' — change whether the answer is correct. 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 configures AS path prepending on an eBGP route to influence inbound traffic. However, traffic from a specific iBGP neighbor still prefers the prepended path. What 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.
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
AS path prepending only affects eBGP path selection; iBGP routers ignore AS path length when comparing paths from different eBGP routers.
AS path prepending affects eBGP path selection but does not affect iBGP path selection because iBGP does not add AS path length to the metric. iBGP routers use IGP metric or other attributes, so prepending is ignored.
Key principle: OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
Answer analysis
Option-by-option breakdown
For each option: why learners choose it and why it is or isn't the right answer here.
- ✓
AS path prepending only affects eBGP path selection; iBGP routers ignore AS path length when comparing paths from different eBGP routers.
- ✗
The prepending was applied with 'set as-path prepend last-as 1', which only prepends the last AS, not the full path.
Why it's wrong here
This would still affect eBGP, but iBGP still ignores AS path length.
- ✗
The iBGP neighbor has 'bgp bestpath as-path ignore' configured, which ignores AS path length entirely.
Why it's wrong here
This would make all paths equal, but the issue is that prepending is not considered.
- ✗
The prepending was done on a route reflector, and the client does not see the prepended AS path.
Why it's wrong here
Route reflectors pass the AS path unchanged, so the client would see it.
Common exam traps
Common exam trap: OSPF can fail even when IP connectivity looks correct
OSPF neighbour formation depends on matching areas, timers, network type, authentication and passive-interface behaviour. Do not choose an answer only because the devices can ping.
Detailed technical explanation
How to think about this question
OSPF questions usually test the details that control adjacency and route selection. Read the neighbour state, area, router ID and interface configuration before deciding what is wrong.
KKey Concepts to Remember
- OSPF neighbours must agree on key parameters.
- Router ID selection can affect neighbour relationships and LSDB output.
- OSPF cost influences the preferred path.
- A route can appear in OSPF information but not become the installed route.
TExam Day Tips
- Check area mismatch first when OSPF adjacency fails.
- Review passive interfaces when a network is advertised but no neighbour forms.
- Use show ip ospf neighbor and show ip route clues carefully.
Key takeaway
OSPF neighbour adjacency depends on matching area, hello/dead timers, network type, and authentication — IP reachability alone is not enough.
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 OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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FAQ
Questions learners often ask
What does this 300-410 question test?
BGP Troubleshooting — This question tests BGP Troubleshooting — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: AS path prepending only affects eBGP path selection; iBGP routers ignore AS path length when comparing paths from different eBGP routers. — AS path prepending affects eBGP path selection but does not affect iBGP path selection because iBGP does not add AS path length to the metric. iBGP routers use IGP metric or other attributes, so prepending is ignored.
What should I do if I get this 300-410 question wrong?
Review OSPF neighbour requirements — matching area type, hello and dead timers, network type, stub flags, and authentication. Study show ip ospf neighbor states (INIT, 2-WAY, FULL). Then practise related 300-410 OSPF questions on adjacency and route selection.
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?
OSPF neighbours must agree on key parameters.
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Last reviewed: Jun 18, 2026
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