300-440 Question 153
Single answerA network engineer has implemented BGP on a router to connect to multiple ISPs for redundancy. To influence inbound traffic by preferring ISP1 over ISP2, the engineer decides to configure a policy. Which configuration approach is most appropriate to achieve this goal?
- A
Apply a route-map on outbound advertisements to ISP1, setting a higher MED value.
- B
Use a route-map on outbound advertisements to ISP2, prepending the AS path multiple times.
- C
Use an inbound route-map on ISP1 to filter specific prefixes.
- D
Set a higher local preference on routes learned from ISP1.
Show answer and explanation
Correct answer: B
Explanation
To influence inbound traffic, the most effective method is to manipulate how your prefixes appear to the external networks. AS path prepending is a common and effective way to make a specific path less attractive to external networks, thereby steering inbound traffic through a preferred ISP. In this scenario, prepending the AS path on advertisements to ISP2 makes it a less desirable path for remote networks, causing them to prefer ISP1 instead.
- A. Incorrect.
Setting a higher MED value on outbound advertisements to ISP1 is incorrect because MED influences inbound path selection from the ISP's perspective, not the inbound traffic received by the local router.
- B. Correct.
Using AS path prepending on outbound advertisements to ISP2 is correct because it makes the path through ISP2 less desirable to remote networks, thereby influencing them to prefer ISP1 for inbound traffic.
- C. Incorrect.
Filtering specific prefixes on inbound routes from ISP1 would not directly influence the inbound traffic flow from external networks; it only affects which routes are accepted by the local router.
- D. Incorrect.
Setting a higher local preference on routes learned from ISP1 influences the outbound path selection of the local router, not the inbound traffic from external networks.