Refer to the exhibit. An engineer configured BGP and wants to select the path from 10.77.255.57 as the best path instead of current best path. Which action resolves the issue?
A.
Configure higher MED to select as the best path.
B.
Configure AS_PATH prepend for the current best path.
C.
Configure AS_PATH prepend for the desired best path.
D.
Configure lower LOCAL_PREF to select as the best path.
Output shows #9 different possible paths. local routers has chosen #6 as best-path. it is asked to what is needed to make path #5 the best-path.
- A - wrong (lower MED is preferred.)
- B - correct (by adding extra AS_PATH makes the current best-path #6 less preferred compared to route #5)
- C - wrong (this is the opposite of B)
- D - wrong (higher LOCAL_PREF is preferred not lower!)
Wondering how to guarantee the #5 path will be chosen as the alternative best rather than any other paths if prepending #6 path as the current best? thanks!
D is correct i think
Explanation
From the output, we learn that the current best path is from 10.57.255.11 (which includes “…valid,
confed-external, best“) and this path is 2 ASes away (64955 65003). Although there are some paths
with only 1 AS away (path from 172.16.254.234 for example) but they were not chosen the best path
so AS_PATH was not used to determine the best path -> Answers A and answer C are not correct.
All the paths in the output have metric of 0 and this is the lowest (best) value for this attribute. If we
configure higher MED then it is less preferred over other paths -> Answer B is not correct.
Only answer D is left but LOCAL_PREF attribute should be configured with higher value to be preferred
so we hope “lower LOCAL_PREF” here means higher value. But this is the best answer
My assumption is that the best path is chosen for the lowest BGP router-id, the lowest is 10.57.255.11 and the second lowest is 10.77.255.57.
If we make 10.57.255.11 less preferred by AS Path Prepending, 10.77.255.57 is going to be selected as best.
All other attributes are the same.
Although B is sort of making sense and according to commen sense, it still hard to convince answer B is correct due to there's other BGP routes with less AS path and the same other attributes.
e.g. metric, localpref but not be choosen the best.
E.g. 172.16.254.234. it's a tricky question.
From the output, we learn that the current best path is from 10.57.255.11 (which includes “...valid, confed-external, best“) and this path is 2 ASes away (64955 65003).
Although there are some paths with only 1 AS away (path from 172.16.254.234 for example) but they were not chosen the best path, so AS_PATH was not used to determine the best path. Answers A and answer C are not correct.
Output shows #9 different possible paths. local routers has chosen #6 as best-path. it is asked to what is needed to make path #5 the best-path.
- A - wrong (lower MED is preferred.)
- B - correct (by adding extra AS_PATH makes the current best-path #6 less preferred compared to route #5)
- C - wrong (this is the opposite of B)
- D - wrong (higher LOCAL_PREF is preferred not lower!)
upvoted 2 times
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