300-410 Question 154
Select 2You are tasked with influencing the outbound routing behavior of your network by ensuring traffic destined for a specific prefix (192.168.100.0/24) takes a preferred path through ISP1, while all other traffic uses ISP2. Which of the following methods would achieve this goal?
- A
Apply a route-map with match statements for the prefix 192.168.100.0/24 and set a higher local preference for routes via ISP1.
- B
Use a prefix list for 192.168.100.0/24 in combination with a BGP AS-PATH prepend on routes advertised to ISP2.
- C
Configure an outbound distribute-list to filter all routes except 192.168.100.0/24 for ISP1.
- D
Apply a route-map to set a lower MED for 192.168.100.0/24 on routes advertised to ISP1.
- E
Implement an inbound route-map on ISP2 to deny the prefix 192.168.100.0/24.
Show answer and explanation
Correct answers: A, B
Explanation
To influence outbound path selection in BGP, the local preference and AS-PATH attributes can be effectively used. Local preference is a powerful attribute for outbound path selection within an AS, while AS-PATH prepending can make specific routes less attractive to other neighbors. These methods ensure that traffic destined for 192.168.100.0/24 prefers ISP1 over ISP2, while all other traffic defaults to ISP2.
- A. Correct.
Correct. Using a route-map to match the specific prefix and set a higher local preference ensures that traffic destined for 192.168.100.0/24 prefers ISP1. Local preference is a strong BGP attribute for influencing outbound path selection.
- B. Correct.
Correct. By applying a prefix list and AS-PATH prepend for 192.168.100.0/24 on ISP2, you make routes from ISP2 less preferred for this specific prefix, ensuring ISP1 is used instead.
- C. Incorrect.
Incorrect. An outbound distribute-list is typically used to control what routes are advertised to a neighbor, not for outbound path manipulation. This would not influence the path selection for the prefix.
- D. Incorrect.
Incorrect. MED is used to influence inbound traffic from a neighbor, not outbound path selection. Applying it on ISP1 would not achieve the desired goal.
- E. Incorrect.
Incorrect. Denying the prefix inbound from ISP2 would prevent your network from learning the route via ISP2 but would not explicitly prefer ISP1. This could lead to suboptimal routing or blackholing.