300-440 Question 152
Single answerA network engineer is tasked with configuring BGP on a router to influence inbound traffic from external peers. The engineer needs to ensure that traffic destined for the company's public IP prefix takes a preferred path through ISP1, rather than ISP2. Which method should the engineer use to achieve this?
- A
Adjust the BGP Local Preference attribute for routes received from ISP1.
- B
Modify the BGP MED (Multi-Exit Discriminator) attribute for routes advertised to ISP1.
- C
Prepend the AS path multiple times for routes advertised to ISP2.
- D
Apply a route-map to set a higher weight for routes learned from ISP1.
Show answer and explanation
Correct answer: C
Explanation
To influence inbound traffic from external peers, AS path prepending is typically used. By adding the local AS number multiple times to the AS path of routes advertised to ISP2, the path through ISP2 appears less desirable to external BGP peers. This encourages them to direct traffic through ISP1 instead, achieving the desired path manipulation for inbound traffic.
- A. Incorrect.
The Local Preference attribute is used to influence outbound traffic within the local AS, not inbound traffic from external peers.
- B. Incorrect.
The MED attribute is used to influence inbound traffic, but it is applicable only when both ISPs belong to the same AS or have an agreement to honor the MED. This is not the most reliable method for influencing inbound traffic across unrelated ISPs.
- C. Correct.
AS path prepending is a common and effective method to influence inbound traffic by making the path through ISP2 less preferred. By artificially lengthening the AS path advertised to ISP2, external peers are more likely to select ISP1 as the preferred path.
- D. Incorrect.
The Weight attribute is specific to the local router and does not influence the path selection of external peers, making it irrelevant for controlling inbound traffic.