The company triples in two years, scale the network

Today the head office has 300 people on a collapsed core pair (CORE1 and CORE2, HSRP and Rapid PVST+) with six access closets. A firewall pair, FW1, runs site-to-site IPsec to three branches, and routing is all static. Over the next 24 months the head office grows to 900 people in a second building across the car park, the branches grow from 3 to 10, and the ERP moves to AWS. The CTO asks: "What breaks, and what would you build?"