DX Public VIF vs. Internet Path Selection
Do we have a blanket prioritization of path for DX over the internet for a public VIF? For example, if a customer advertises a pair of /23 through their ISP, and a single /22 over the DX will we send the traffic to them over the internet, or over the DX?
The prefix example is contrived, but does serve to illustrate the larger question- do we weight connection type of DX over public internet routing irrespective of all other traditional routing considerations? The FAQ doc lists the following: "AS_PATH is used to determine the routing path, and AWS Direct Connect is the preferred path for traffic sourced from Amazon. Only public ASNs are used internally for route selection.", but being in the AS_PATH section makes me want to double check.
Thanks!
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asked 7 years ago609 views
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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?