350-401 Question 478
Single answerA network engineer is troubleshooting a BGP routing issue in which a specific prefix is not being selected as the best path. The BGP table shows multiple valid paths for the prefix, but the path through Router A is not being chosen despite appearing to be the most direct. Upon further investigation, the engineer observes the following attributes for the prefix on Router A: Weight = 0, Local Preference = 100, AS Path = 5, MED = 0, and Origin = IGP. Based on this information, what is the likely reason Router A's path is not being selected?
- A
The AS Path attribute has a higher value compared to other paths.
- B
The Weight attribute is set to 0, giving it the lowest priority.
- C
The MED value of 0 is less preferred when compared to other paths' MED values.
- D
Another path has a higher Local Preference than the path through Router A.
Show answer and explanation
Correct answer: D
Explanation
BGP uses a specific order to evaluate attributes when determining the best path. Local Preference is one of the first attributes considered and takes precedence over attributes like AS Path and MED. In this scenario, another path likely has a higher Local Preference than the path through Router A, leading to its selection as the best path. Attributes like Weight, AS Path, and MED are evaluated later in the process and do not influence the decision in this case.
- A. Incorrect.
The AS Path attribute has a higher value compared to other paths, but this does not necessarily mean it will be the deciding factor. BGP evaluates attributes in a specific order, and AS Path is not the first attribute considered.
- B. Incorrect.
The Weight attribute is set to 0, but Weight is a Cisco-specific attribute and is only relevant locally. If all paths have the same Weight (default 0), it does not influence the decision unless explicitly configured.
- C. Incorrect.
A MED value of 0 is considered the most preferred when MED is compared. This is not the reason Router A's path is not selected.
- D. Correct.
Local Preference is evaluated before AS Path and MED in the BGP best-path selection process. If another path has a higher Local Preference, it will be selected over the path through Router A.