The answer is that the next-hop IP of the eBGP route is not reachable via IGP, making the route invalid. This occurs because iBGP, by default, does not change the next-hop attribute when advertising routes learned from an eBGP peer; the next-hop remains the eBGP neighbor’s interface IP. For an iBGP route to be considered valid and installed in the routing table, that next-hop address must be reachable through an Interior Gateway Protocol (IGP) route—if it is missing, the route is marked as not best and is not advertised. On the Cisco CCNP ENARSI 300-410 exam, this scenario tests your understanding of BGP next-hop processing and the common pitfall where next-hop-self is omitted on the iBGP speaker. A frequent trap is assuming an established iBGP session guarantees route propagation, but the real requirement is recursive IGP reachability to the next-hop. Memory tip: “iBGP trusts the IGP—if the next-hop is missing, the route is dismissed.”
300-410 Route Redistribution Practice Question
This 300-410 practice question tests your understanding of route redistribution. Examine the command output carefully: the correct answer depends on what the output actually shows, not on general recall alone. 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 BGP between two routers in the same AS. The iBGP session is established, but routes learned from the eBGP neighbor are not advertised to the iBGP peer. The 'show ip bgp' output shows the routes as valid but not best. 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: "best"
Why it matters: Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
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 of the eBGP route is not reachable via IGP, making the route invalid.
For iBGP, the next-hop must be reachable via IGP. If the next-hop is not reachable (e.g., no IGP route for the eBGP peer's interface IP), the route is considered invalid and not installed. This is a common edge case where next-hop-self is not configured.
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.
✗
The 'synchronization' rule is enabled and the IGP does not carry the prefix.
Why it's wrong here
Synchronization is disabled by default in modern IOS.
✓
The next-hop IP of the eBGP route is not reachable via IGP, making the route invalid.
Why this is correct
iBGP requires next-hop reachability; without next-hop-self, the next-hop is the eBGP peer's IP.
Clue confirmation
The clue words "best", "most likely" in the question point toward this answer.
The 'maximum-paths' command is set to 1, preventing load balancing.
Why it's wrong here
Maximum-paths affects load balancing, not route advertisement.
✗
The 'bgp bestpath med missing-as-worst' command causes the route to be considered worse.
Why it's wrong here
This affects best path selection but not next-hop reachability.
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.
Related glossary terms
Concepts from this question explained
These glossary pages explain the core terms tested in this 300-410 question in full detail.
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.
Route Redistribution — This question tests Route Redistribution — OSPF neighbours must agree on key parameters..
What is the correct answer to this question?
The correct answer is: The next-hop IP of the eBGP route is not reachable via IGP, making the route invalid. — For iBGP, the next-hop must be reachable via IGP. If the next-hop is not reachable (e.g., no IGP route for the eBGP peer's interface IP), the route is considered invalid and not installed. This is a common edge case where next-hop-self is not configured.
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: "best", "most likely". Signals that multiple options may be partially correct. Choose the option that most directly solves the exact problem described, not the one that sounds most complete.
What is the key concept behind this question?
OSPF neighbours must agree on key parameters.
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