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Thanks! I am more curious about the physical aspect of it. How do you get multiple switches to share an IP? What determines with switch forwards the packets? Like, simply, what wires connect to what?

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Your switches run a routing protocol (almost always BPG) and your load balancers all announce the same IP to them. This results in a routing table with many next hops for the LB IP, and traffic is distributed across them using ECMP, typically with some resilient hashing scheme so that a given flow will always go to the same physical LB, even if there are topology changes due to LB or switch failures.

At the edges, your switches or border routers announce the IP to all your peers and transit ISPs.

Yes, this. The beauty of the Unimog setup we run in production is we are resilient to ECMP rehash events — each server acting as a load balancer has access to state (backed by Consul) that tells it which flow belongs to which application server.

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