300-420 Question 126
Single answerA network engineer is tasked with configuring a BGP route filter on a border router to prevent specific prefixes from being advertised to an external BGP (eBGP) peer. The engineer needs to ensure that no unintended prefixes are impacted while maintaining control over the advertised routes. Which of the following methods would be most appropriate to achieve this?
- A
Use a prefix list to explicitly permit the required prefixes and deny all others.
- B
Apply a route-map that sets the local preference attribute for the prefixes to be blocked.
- C
Enable BGP soft reconfiguration to dynamically filter prefixes without requiring explicit configuration.
- D
Use an access control list (ACL) to block the prefixes during BGP updates.
Show answer and explanation
Correct answer: A
Explanation
Prefix lists are specifically designed for BGP route filtering and provide a precise mechanism to control which prefixes are advertised to or received from a BGP peer. By permitting only the required prefixes and denying others, the engineer can ensure that no unintended prefixes are impacted. Other methods, such as modifying attributes or using ACLs, are not appropriate for this task.
- A. Correct.
Using a prefix list is the most appropriate method for filtering specific prefixes in BGP. It allows precise control over which prefixes are advertised by explicitly permitting the required ones and denying the unwanted ones.
- B. Incorrect.
Setting the local preference attribute is used to influence route selection within an autonomous system, not for filtering prefixes between BGP peers.
- C. Incorrect.
BGP soft reconfiguration is used to enable dynamic updates of routing information without clearing BGP sessions, but it does not directly filter prefixes.
- D. Incorrect.
Access control lists (ACLs) are primarily used for packet filtering and are not designed to handle BGP route filtering.