Question 1,318 of 2,152

NetFlow and Flexible NetFlow →hardMultiple ChoiceObjective-mapped

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.

300-410 NetFlow and Flexible NetFlow Practice Question

This 300-410 practice question tests your understanding of netflow and flexible netflow. 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 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.

Question 1hardmultiple choice

Open the full BGP breakdown →

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.
With synchronization enabled, BGP will not advertise iBGP routes unless the route is also present in the IGP. This is a common edge case in older configurations.
B
The iBGP session is using a loopback interface that is not reachable via the IGP.
Why wrong: The scenario states the next-hop is reachable via IGP, so this is not the issue.
C
The BGP table shows the route as 'valid' but 'not best' due to a higher local preference.
Why wrong: If the route is not best, it would not be advertised, but the scenario does not mention any path selection issues.
D
The router is configured with 'bgp bestpath med missing-as-worst', which affects MED comparison but not advertisement.
Why wrong: This command affects best path selection but does not prevent advertisement if the route is best.

Full breakdown with real-world context →

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

✓

BGP synchronization is enabled, and the route is not present in the IGP routing table, so BGP does not advertise it to iBGP peers.

In iBGP, the next-hop-self command is often required because the next-hop of eBGP routes is the eBGP neighbor's IP, which may not be reachable by iBGP peers. However, even if the next-hop is reachable, BGP will not advertise routes to an iBGP neighbor if the route is learned from another iBGP peer (split horizon rule). Additionally, if the route is learned from an eBGP peer, the next-hop must be reachable via IGP or a static route. A common edge case is when the next-hop is reachable, but the BGP table shows the route as 'not advertised to any peer' because the router is not synchronized with the IGP (if synchronization is enabled) or because the route is not in the routing table due to a missing network statement or redistribution.

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.

✓
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 this is correct
With synchronization enabled, BGP will not advertise iBGP routes unless the route is also present in the IGP. This is a common edge case in older configurations.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The iBGP session is using a loopback interface that is not reachable via the IGP.
Why it's wrong here
The scenario states the next-hop is reachable via IGP, so this is not the issue.
✗
The BGP table shows the route as 'valid' but 'not best' due to a higher local preference.
Why it's wrong here
If the route is not best, it would not be advertised, but the scenario does not mention any path selection issues.
✗
The router is configured with 'bgp bestpath med missing-as-worst', which affects MED comparison but not advertisement.
Why it's wrong here
This command affects best path selection but does not prevent advertisement if the route is best.

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.

Trap categories for this question

Command / output trap
This command affects best path selection but does not prevent advertisement if the route is best.
Scenario analysis trap
The scenario states the next-hop is reachable via IGP, so this is not the issue.

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?

NetFlow and Flexible NetFlow — This question tests NetFlow and Flexible NetFlow — OSPF neighbours must agree on key parameters..

What is the correct answer to this question?

The correct answer is: BGP synchronization is enabled, and the route is not present in the IGP routing table, so BGP does not advertise it to iBGP peers. — In iBGP, the next-hop-self command is often required because the next-hop of eBGP routes is the eBGP neighbor's IP, which may not be reachable by iBGP peers. However, even if the next-hop is reachable, BGP will not advertise routes to an iBGP neighbor if the route is learned from another iBGP peer (split horizon rule). Additionally, if the route is learned from an eBGP peer, the next-hop must be reachable via IGP or a static route. A common edge case is when the next-hop is reachable, but the BGP table shows the route as 'not advertised to any peer' because the router is not synchronized with the IGP (if synchronization is enabled) or because the route is not in the routing table due to a missing network statement or redistribution.

What should I do if I get this 300-410 question wrong?

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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