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It does, but it's not very useful, as you cannot have a VIP span multiple AZs and typically a spontaneous instance failure is correlated with an AZ problem. ENIs (and IPs) are tied to a subnet, which is tied to an AZ. Even the example only utilizes a single availability zone. While there may be a tiny bit of additional redundancy added by doing this, you still need a way to fail over to another availability zone.

I would love a way to do multicast UDP in a VPC (i.e. for discovery). I've been hopeful for years..

interesting, I can't find anything that supports this. Do you have a link to a doc?

edit: or are you referring to using ENIs?

Sure, here you go. Basically, you can set up a private subnet, from which you can use any IP.

http://aws.amazon.com/articles/2127188135977316

It looks like this is using APIs not ARP to move the IP address; am I wrong?

that's correct as far as I understand the capabilities in VPC.

This isn't ARP per se as one would traditionally expect it. This is the networking in VPC handling the movement of the ENI.