VPC Peering
A direct network connection between two VPCs, which is simple, cheap and non-transitive — the last property being the one that shapes topology.
Peering connects two VPCs so instances can reach each other by private address. It is low-latency, avoids the internet, and requires non-overlapping ranges.
Non-transitivity is the constraint. If A peers with B and B peers with C, A cannot reach C. Full connectivity between N VPCs therefore requires N(N−1)/2 peerings — ten VPCs is 45 connections, each with route table entries in both directions. It does not scale.
Which is why hub-and-spoke topologies using a transit gateway (or hub VPC) replace peering beyond a handful of networks: each VPC connects once to the hub, routing is centralised, and adding a network is one attachment rather than N.
Peering remains right for a small number of stable, high-volume connections where the transit gateway's per-gigabyte charge would exceed its convenience.