The OSI stack is a great tool when working with complex designs. Every layer in the OSI stack must be considered within the design and have a purposeful answer. Designs always start at the physical layer, working up the stack from the bottom to the top. See the following diagram. Many load balancers today work at all layers of the OSI stack. Back to the question: how are multiple load balancers physically connected to multiple servers creating multiple ingress and egress paths? Multiple switches may also be required. Today many load balancers combine the port density of switches, the routing capability of routers, and the logical functions of load balancers, all in a single device simplifying designs and saving a bit of budget money.
The web layer and application layers can often be collapsed into the same server. From a security perspective, this...