Your company has two DX locations. You need to configure one link as passive. What should you configure in your router to set that link as the passive link.
A.
Set a higher MED.
B.
Configure AS_PATH Prepending on the link.
C.
Advertise a network with a higher CIDR.
D.
Call your service provider and have the ASN changed for that link.
Suggested Answer:B🗳️
You should configure AS_PATH prepending on the link. A higher CIDR is the same as a more specific prefix, which will make the link more preferred. A higher MED will make the path less preferred, but this is not the preferred method to accomplish this. Changing your ASN will not help. Configuring AS_PATH Prepending is the preferred method of AWS to configure an Active-Passive configuration with Direct Connect.
A, B and C could all potentially make it happen. https://aws.amazon.com/blogs/networking-and-content-delivery/creating-active-passive-bgp-connections-over-aws-direct-connect/
MED and AS-Path both can be used for the route manipulation. MED is not avaible on the public VIF but it is not mentioned here.
Bigger CIDR (if it means it is less specific) will also move the traffic onto the other link
I have no clue which answer is correct... Ideas guys?
Higher CIDR means more subnet bits are used, or more specific prefixes. It will make this route more preferable. So C won't work.
BTW, only see higher CIDR term used in AWS.
basically A and B are the same, it is that in BGP world AS_PATH is considered first. however - question mentioned different locations. if these are in different regions - AS_PATH does not work
https://repost.aws/knowledge-center/active-passive-direct-connect
best choice (which is not provided) is BGP community tags
i would say MED
Answer A should be right. It’s saying to set a higher MED, which for sure will make the link less preferable!
Answer B should be wrong: It’s saying to do SOME config for AS_PATH. Sounds like the AS_PATH config is/was missing. If you configure same or shorter AS_PATH, it will not make the link less preferable.
Higher MED- make it worse - make it passive
as-path prepending - make it worse too - make it passive
Higher CIDR - like from /24 to /25 - make it stronger - make it active (wrong)
So the question here what is the better answer (med or as-path prepending).
I don't see any information about what type of BGP we are using - eBGP or iBGP, but in general as-path prepending is the better choce, because MED could be ignored in some cases.
So. "B"
Question is incomplete. You must influence your peer to prefer one link over another (AS prepending), but, in order to avoid asymmetric routing, you must influence your router to prefer the same path you are enforcing your peer to use (Local Preference).
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