Question 842 of 2,152

MPLS L3VPN →hardMultiple ChoiceObjective-mapped

Quick Answer

The answer is that BGP synchronization is enabled and the prefix is not present in the IGP, so BGP does not advertise it. In BGP, the synchronization rule requires that a prefix learned via an eBGP neighbor must also exist in the Interior Gateway Protocol (IGP) before it is advertised to iBGP peers, ensuring the IGP can route traffic across the autonomous system. When this rule is enabled on a Provider Edge router in an MPLS L3VPN, a customer prefix injected via the `network` command will not be advertised to BGP neighbors unless that exact prefix is also reachable through the IGP, even if it appears in the routing table. This scenario tests your understanding of how BGP synchronization interacts with prefix advertisement in the Cisco CCNP ENARSI 300-410 exam, often appearing as a subtle trap where candidates overlook the synchronization default state—though disabled in MPLS L3VPN, it can be manually enabled. A common memory tip is "Sync or sink": if synchronization is on, the prefix must swim in the IGP or it sinks from BGP advertisement.

300-410 MPLS L3VPN Practice Question

This 300-410 practice question tests your understanding of mpls l3vpn. 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 on a PE router in an MPLS L3VPN to advertise a customer prefix. The engineer notices that the prefix is not being advertised to the BGP neighbor, even though the prefix is in the routing table and the BGP session is up. The engineer checks the BGP configuration and sees that the 'network' command is configured correctly. The engineer also verifies that the prefix is not being filtered by any route-map. 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.

Question 1hardmultiple choice

Open the full BGP breakdown →

A
BGP synchronization is enabled, and the prefix is not present in the IGP, so BGP does not advertise it.
Correct. If synchronization is enabled, BGP will not advertise a prefix unless it is also in the IGP. This is a common edge case.
B
The 'network' command is missing the 'mask' keyword, so BGP advertises the classful network instead.
Why wrong: Incorrect. The 'network' command without a mask advertises the classful network, but the prefix would still be advertised, just with a different prefix length.
C
The BGP neighbor is configured with 'next-hop-self', which prevents the prefix from being advertised.
Why wrong: Incorrect. 'next-hop-self' changes the next-hop but does not prevent advertisement.
D
The prefix is a connected route, and BGP requires the 'network' command to include the 'route-map' to advertise connected routes.
Why wrong: Incorrect. The 'network' command can advertise connected routes without a route-map, as long as the prefix matches exactly.

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 prefix is not present in the IGP, so BGP does not advertise it.

In BGP, the 'network' command advertises a prefix only if it exists in the routing table with the exact same prefix length and next-hop. A common edge case is that the prefix in the routing table has a different prefix length (e.g., a /24 is in the routing table but the 'network' command specifies a /25) or the prefix is learned via a different protocol with a different administrative distance. Another edge case is that the BGP synchronization rule (if enabled) requires the prefix to be in the IGP before it is advertised. In MPLS L3VPN, synchronization is disabled by default, but if it is enabled, the prefix must be in the IGP. The most likely explanation is that synchronization is enabled and the prefix is not in the IGP.

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 prefix is not present in the IGP, so BGP does not advertise it.
Why this is correct
Correct. If synchronization is enabled, BGP will not advertise a prefix unless it is also in the IGP. This is a common edge case.
Clue confirmation
The clue word "most likely" in the question point toward this answer.
Related concept
OSPF neighbours must agree on key parameters.
✗
The 'network' command is missing the 'mask' keyword, so BGP advertises the classful network instead.
Why it's wrong here
Incorrect. The 'network' command without a mask advertises the classful network, but the prefix would still be advertised, just with a different prefix length.
✗
The BGP neighbor is configured with 'next-hop-self', which prevents the prefix from being advertised.
Why it's wrong here
Incorrect. 'next-hop-self' changes the next-hop but does not prevent advertisement.
✗
The prefix is a connected route, and BGP requires the 'network' command to include the 'route-map' to advertise connected routes.
Why it's wrong here
Incorrect. The 'network' command can advertise connected routes without a route-map, as long as the prefix matches exactly.

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
Incorrect. The 'network' command without a mask advertises the classful network, but the prefix would still be advertised, just with a different prefix length.

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?

MPLS L3VPN — This question tests MPLS L3VPN — 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 prefix is not present in the IGP, so BGP does not advertise it. — In BGP, the 'network' command advertises a prefix only if it exists in the routing table with the exact same prefix length and next-hop. A common edge case is that the prefix in the routing table has a different prefix length (e.g., a /24 is in the routing table but the 'network' command specifies a /25) or the prefix is learned via a different protocol with a different administrative distance. Another edge case is that the BGP synchronization rule (if enabled) requires the prefix to be in the IGP before it is advertised. In MPLS L3VPN, synchronization is disabled by default, but if it is enabled, the prefix must be in the IGP. The most likely explanation is that synchronization is enabled and the prefix is not in the IGP.

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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