Exam ANS-C00 topic 1 question 172 discussion

Exam question from Amazon's ANS-C00

Question #: 172
Topic #: 1

You have 4 Direct Connect connections from your datacenter. Site A advertises 172.16.0.0/16 AS 65000, Site B advertises 172.16.0.128/25 AS 65000 65000
65000, Site C advertises 172.0.0.0/8 AS 65000 and Site D advertises 172.16.0.0/24 AS 65000. Which site will AWS choose to reach your network?

A. Site A: 172.16.0.0/16 AS 65000
B. Site B: 172.16.0.128/25 AS 65000 65000 65000
C. Site C: 172.0.0.0/8 AS 65000
D. Site D: 172.16.0.0/24 AS 65000

Show Suggested Answer

Suggested Answer: B 🗳️
172.16.0.128/25 AS 65000 65000 65000. The most specific prefix is always the first choice for BGP routing. Also, AWS will not accept an advertisement of a network less than /16.

by awspro2021 at March 11, 2021, 7:28 a.m.

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ChauPhan

Highly Voted 3 years, 8 months ago

B. Site B: 172.16.0.128/25 AS 65000 65000 65000 Because it is most specific routes AWS DX usually choose the most specific route. If 02 routes are same length, they will consider in priority: local preference, AS_PATH and MED. Tips: We Love Oranges AS Oranges Mean Pure Refreshment This translates to: Weight, Local Pref, Originate, AS_Path, Origin, MED, Paths, RouterID

upvoted 19 times

StelSen

3 years, 7 months ago

I like the TIPS ChauPhan.

upvoted 2 times

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wahlbergusa

Highly Voted 3 years, 7 months ago

I wish when anyone claims something they can back it up with an official+public AWS documentation. That is the best way. It is quite clear here : https://docs.aws.amazon.com/vpc/latest/userguide/VPC_Route_Tables.html#route-tables-priority . Everyone please read "Route Priority" section and you will see that the answer is obviously B.

upvoted 5 times

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anchen

Most Recent 2 years, 8 months ago

in the bgp path selection, the first option would be the prefix length. the longer prefix always win, but it has a condition if the address try to route falls into multiple subnet. in this question, i do think the answer is B. However, i think the question is confusion. we think the answer is B, because we assume the IP AWS want to route to falls under 172.16.0.128/25. what if the destination is: 172.16.0.1? it means the site D would win, because it does not exist on the /25 range of .128.

upvoted 2 times

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