Network infrastructure of large networks

Hi,

Currently, is it possible to run a large network with Free Software?

If a large organization would like to run only Free Software, how would their network infrastructure be?

Which devices and software should they use to replace their proprietary state-of-the-art network appliances?
I mean stackable 48-port switches which have optical uplinks, high-performance routers with optical and other interfaces, powerful multi-feature firewalls, and so on.

Which existing hardware and software would you use?
Let's think we are millionaires.

Thank you.