hardMultiple ChoiceObjective-mapped
300-410 Practice Question: An engineer configures BGP on a PE router in an…
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?
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.
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.
- ✗
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.
Go deeper
Related to this question
Learn chapter
Introduction to ENARSI Exam and Network Fundamentals
Key term
LDP Protocol
LDP, or Label Distribution Protocol, is a protocol that routers use to automatically exchange labels that enable MPLS (Multiprotocol Label Switching) to create fast, efficient paths for data packets across a network.
Key term
MPLS Layer 3 VPN
A technology that uses Multiprotocol Label Switching to create secure, scalable virtual private networks that connect multiple sites at the network layer, where the service provider manages routing between customer sites.
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Written by Johnson Ajibi, MSc IT Security
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