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As per standard BGP routing - longest prefix match will always apply first. So in your example, if the prefix being seen is a /23 over the Internet vs a path to the same IP's within a /22 announcement over DX - we will prefer the Internet.
If however we see the same /23 prefix over both the Internet and DX then we prefer the DX path.
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This is a Question: if you add your own public CIDR of /23 (just an example: 4.4.0.0/23) to the "Prefixes you want to advertise", but advertise a /24 (4.4.0.0/24) through the BGP, what would AWS advertise within the region? will it be the longer /23 prefix added or the smaller /24 advertised through BGP?