Why does a second network interface require a separate VPC (subnet in a separate VPC) to attach to?
The reason has to do partly with the way routes are inherited by network interfaces.
NICs automatically inherit default routes from the global routing table defined at the VPC Level.
The question then arises - if you add a second NIC, what is the advantage of it inheriting exactly the same routes?
Since the idea of a second NIC is typically to provide additional (Secondary) network routing, it makes sense to attach it to a second VPC that is different from the first.
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