300-420 Question 29
Single answerA company is configuring route filtering on a border router to prevent specific routes from being advertised to an external BGP (eBGP) neighbor. They want to block routes from the 192.168.1.0/24 network while allowing all other networks to be advertised. Which method would be the most appropriate to achieve this?
- A
Use a distribute-list with a prefix list denying 192.168.1.0/24 and permitting all other networks.
- B
Configure a static route for 192.168.1.0/24 pointing to null0.
- C
Apply an outbound route-map that matches the 192.168.1.0/24 network and denies it, while permitting all other traffic.
- D
Use an access control list (ACL) to block 192.168.1.0/24 traffic on the interface.
Show answer and explanation
Correct answer: C
Explanation
When filtering routes in BGP, an outbound route-map is the most appropriate method for selectively blocking specific prefixes from being advertised to a neighbor. In this scenario, the route-map can match the 192.168.1.0/24 network and deny it, while permitting all other routes, ensuring effective route filtering.
- A. Incorrect.
A distribute-list can be used for route filtering, but it is typically applied in scenarios involving IGPs (like OSPF or EIGRP), not for BGP route advertisement control.
- B. Incorrect.
A static route to null0 is useful for route suppression within the local routing table but does not control route advertisements to an external neighbor.
- C. Correct.
An outbound route-map is the correct method for filtering BGP advertisements. It allows specific prefixes to be denied (such as 192.168.1.0/24) while permitting all others.
- D. Incorrect.
An ACL cannot be directly applied to filter BGP route advertisements. ACLs are used for traffic filtering, not route filtering.