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Exam ANS-C00 topic 1 question 164 discussion

Exam question from Amazon's ANS-C00
Question #: 164
Topic #: 1
[All ANS-C00 Questions]

You have two Direct Connect connections and two VPN connections to your network. Site A is VPN 10.1.0.0/24 AS 65000 65000, Site B is VPN 10.1.0.252/30 AS
65000, Site C is DX 10.0.0.0/8 AS 65000 and Site D is DX 10.0.0.0/16 AS 65000 65000 65000. Which site will AWS choose to reach your network?

  • A. Site A: VPN 10.0.1.0/24 AS 65000 65000
  • B. Site B: VPN 10.0.1.252/30 AS 65000 65000 65000
  • C. Site C: DX 10.0.0.0/8 AS 65000
  • D. Site D: DX 10.0.0.0/16
Show Suggested Answer Hide Answer
Suggested Answer: B 🗳️
Site B, the most specific prefix always wins.

Comments

Chosen Answer:
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JamesTR
Highly Voted 2 years, 6 months ago
Impossible to answer without knowing destination IP
upvoted 8 times
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Huntkey
Highly Voted 2 years, 7 months ago
Seriously, I think the people who come up this question don't know about networking. This question depends on what IP AWS uses to reach your site... Also, who has site with just a /30 prefix?
upvoted 8 times
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slackbot
Most Recent 1 year ago
:D no destination IP, how should we know :D also, answer B contains 3 ASN, while in the question there is 1 ASN
upvoted 1 times
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ongre_
1 year, 9 months ago
Selected Answer: B
Longest prefix always wins
upvoted 2 times
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haint
1 year, 10 months ago
DX will be preferred over VPN connection. 10.0.0.0/16 route is more specific than 10.0.0.0/8, thus D should be a right answer.
upvoted 1 times
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sayed_2908
2 years, 5 months ago
Agree that DX have priority over VPN but keep in mind that more specific prefix always win.
upvoted 3 times
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JoMainAWS
2 years, 5 months ago
what???? hahaha
upvoted 2 times
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walkwolf3
2 years, 5 months ago
B ===Route selection Longest prefix match applies. If the prefixes are the same, then the virtual private gateway prioritizes routes as follows, from most preferred to least preferred: BGP propagated routes from an AWS Direct Connect connection Manually added static routes for a Site-to-Site VPN connection BGP propagated routes from a Site-to-Site VPN connection For matching prefixes where each Site-to-Site VPN connection uses BGP, the AS PATH is compared and the prefix with the shortest AS PATH is preferred. https://docs.aws.amazon.com/vpn/latest/s2svpn/VPNRoutingTypes.html#vpn-route-priority
upvoted 3 times
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zenfox
2 years, 6 months ago
question is bad and answer are different lol I will go with D DX is better than VPN. Also C has one as-path vs D that has none in the answer but 3 in the question
upvoted 2 times
zenfox
2 years, 6 months ago
plus D has more specific route as well. /16 is better than /8
upvoted 1 times
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ChauPhan
2 years, 6 months ago
https://docs.aws.amazon.com/vpn/latest/s2svpn/VPNRoutingTypes.html#vpn-route-priority When a virtual private gateway receives routing information, it uses path selection to determine how to route traffic. Longest prefix match applies. If the prefixes are the same, then the virtual private gateway prioritizes routes as follows, from most preferred to least preferred: BGP propagated routes from an AWS Direct Connect connection Manually added static routes for a Site-to-Site VPN connection BGP propagated routes from a Site-to-Site VPN connection For matching prefixes where each Site-to-Site VPN connection uses BGP, the AS PATH is compared and the prefix with the shortest AS PATH is preferred.
upvoted 2 times
ChauPhan
2 years, 6 months ago
It will choose B because it is most specific route B. Site B: VPN 10.0.1.252/30 AS 65000 65000 65000
upvoted 2 times
ChauPhan
2 years, 6 months ago
If 02 routes with same length, then AWS will choose DX instead of VPN
upvoted 1 times
abc321
2 years, 6 months ago
DX routes will be preferred over VPN routes. Even though B has specific route it wont be used as DX routes C and D will be preferred. Ans would be D as its more specific than C.
upvoted 1 times
ptpho
2 years, 6 months ago
Agreed with D. DCX > VPN and more specific > ASP
upvoted 1 times
ptpho
2 years, 6 months ago
changed my opinion. most specific route always selected. This is logical and we dont care about /30 because this is a site, not AWS VPC CIDR. So ans is B
upvoted 1 times
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ChauPhan
2 years, 6 months ago
If DX routes are same length, then it will use local preference communities, AS_PATH to for route priority.
upvoted 1 times
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dev62
2 years, 6 months ago
ANS should be D. As both A & B are not the same CIDR as in the question. C -is larger one. So, D more specific Route is /16.
upvoted 1 times
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eeghai7thioyaiR4
2 years, 6 months ago
That question is aweful However, let's gamble B: no. "Your network" is probably something more than a /30 A: no. VPN A has 10.1.0.0/24, while the answer states "10.0.1.0/24" This leaves C and D .. Take a random pick ?
upvoted 3 times
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Community vote distribution
A (35%)
C (25%)
B (20%)
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