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Exam ANS-C00 topic 1 question 56 discussion

Exam question from Amazon's ANS-C00
Question #: 56
Topic #: 1
[All ANS-C00 Questions]

A network engineer is managing two AWS Direct Connect connections. Each connection has a public virtual interface configured with a private ASN. The engineer wants to configure active/passive routing between the Direct Connect connections to access Amazon public endpoints. What BGP configuration is required for the on-premises equipment? (Choose two.)

  • A. Use Local Pref to control outbound traffic.
  • B. Use AS Prepending to control inbound traffic.
  • C. Use eBGP multi-hop between loopback interfaces.
  • D. Use BGP Communities to control outbound traffic.
  • E. Advertise more specific prefixes over one Direct Connect connection.
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Suggested Answer: AE 🗳️
by ohcan at April 28, 2019, 7:25 p.m.
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clark
Highly Voted 3 years, 9 months ago
A and E are the correct answers. B is incorrect. https://docs.aws.amazon.com/directconnect/latest/UserGuide/getting_started.html#createvirtualinterface. "BGP information: A public or private Border Gateway Protocol (BGP) Autonomous System Number (ASN) for your side of the BGP session. If you are using a public ASN, you must own it. If you are using a private ASN, it must be in the 64512 to 65535 range. Autonomous System (AS) prepending does not work if you use a private ASN for a public virtual interface. An MD5 BGP authentication key. You can provide your own, or you can let Amazon generate one for you. (Public virtual interface only) Prefixes you want to advertise: Public IPv4 routes or IPv6 routes to advertise over BGP. You must advertise at least one prefix using BGP, up to a maximum of 1,000 prefixes."
upvoted 42 times
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Rim007
Highly Voted 3 years, 9 months ago
A. Use Local Pref to control outbound traffic. E. Advertise more specific prefixes over one Direct Connect connection.
upvoted 9 times
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etarga
Most Recent 2 years, 6 months ago
Selected Answer: AE
Correct answers A & E
upvoted 1 times
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Joshua555
3 years ago
Selected Answer: AE
AWS has a software limitation - they can't prepending ASN for private ASN for a public interface. so A, E
upvoted 1 times
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nklocal
3 years, 1 month ago
A & E, The local preference is local to the AS, and it is for outbound routing.
upvoted 1 times
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douglasaws
3 years, 1 month ago
Selected Answer: AB
A and B... if you prepend the routes you advertise via one Direct Connect, it will make the inbound traffic flows via the link with no preppend...so B is right if you set local preference in one neighbor higher, the outbound will go trough it, A is right E could work, but is a bad solution
upvoted 1 times
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sapien45
3 years, 4 months ago
https://aws.amazon.com/premiumsupport/knowledge-center/dx-create-dx-connection-from-public-vif/ Longer Prefixe is definitely an option for private ASN. But, Local Preference is listed as an option for Public ASN ...
upvoted 1 times
sapien45
3 years, 4 months ago
Outbound traffic : from Datacenter to AWS Local prefernces works only between iBGP perrs, not eBGP peers , in other words within the same ASN On the datacenter side there is only one ASN https://aws.amazon.com/blogs/networking-and-content-delivery/creating-active-passive-bgp-connections-over-aws-direct-connect/
upvoted 2 times
jerac58653
3 years, 1 month ago
This is not correct. The Local Preference is not attached to eBGP updates (but there is no neede) and it only stays within the AS (iBGP). It is still the main attribute used to influence the preference for the outbound traffic from a customer router or ASN if the connections are terminated on different routers. When the router choses best path for outbound traffic, it first check the weight if it is Cisco, then local preference of the routes. As this is local setting only on customer managed equipment, it can be used with no problem, no AWS related restrictions.
upvoted 1 times
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ceros399
3 years, 6 months ago
Selected Answer: AE
A & E; Afer reading clark.
upvoted 3 times
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JamesTR
3 years, 8 months ago
AE https://aws.amazon.com/premiumsupport/knowledge-center/dx-create-dx-connection-from-public-vif/
upvoted 2 times
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Thiagosilvanetwork
3 years, 8 months ago
Does someone already try to use "Private-VIFs" + Private ASN + AS-Prepending? Is that works? I have not found anything related to "Private-Vifs" when using with AS-Prepending
upvoted 1 times
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MaikM
3 years, 8 months ago
Inbound and outbound in reference to AWS or to on-premises network ?
upvoted 1 times
Huntkey
3 years, 8 months ago
Well, guess what, you are taking an AWS exam. Of course, it is from the AWS' perspective. A and E make total sense for that.
upvoted 1 times
JamesTR
3 years, 8 months ago
Question says : "What BGP configuration is required for the on-premises equipment?" So inbound and outbound means in and out from on-premises customer network.
upvoted 3 times
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PeppaPig
3 years, 9 months ago
A&E https://aws.amazon.com/premiumsupport/knowledge-center/dx-create-dx-connection-from-public-vif/ If you're using a private ASN: AS prepending doesn't work if you use a private ASN for a public virtual interface
upvoted 5 times
Kentik
3 years, 8 months ago
I looked into this one and your correct, if you have a private AS on a public VF the AS-prepend wont work, the answer is A and E
upvoted 1 times
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leotp
3 years, 9 months ago
A,B as below: https://forums.aws.amazon.com/thread.jspa?threadID=264393
upvoted 1 times
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sensor
3 years, 9 months ago
A,B ref:https://aws.amazon.com/premiumsupport/knowledge-center/dx-create-dx-connection-from-public-vif/
upvoted 1 times
sensor
3 years, 9 months ago
Correction: in the provided link for the private ASN use case more specific prefix must be advertized from customer gateway: If you're using a private ASN: Confirm that your customer gateway is advertising the longer prefix on your primary connection. For example, if you're advertising prefix X.X.X.0/24, then your customer gateway can advertise two prefixes (X.X.X.0/25 and X.X.X.128/25) on your primary connection and prefix X.X.X.0/24 on your secondary connection.
upvoted 3 times
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shafeeqsa
3 years, 9 months ago
And B , Communities are used with direct connect to control inbound traffic..
upvoted 1 times
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kvirk
3 years, 9 months ago
A, E is correct.
upvoted 2 times
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luckymuki
3 years, 9 months ago
Answer is AE. https://forums.aws.amazon.com/message.jspa?messageID=813305 "Public VIF : AS path prepend WILL NOT be honored if you are using a private ASN"
upvoted 4 times
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