At the moment, network switching and routing with open source products + commodity servers can't scale like Cisco and Juniper...if you want to have a fairly standard network (OSPF, BGP, etc).

Google, for example, is able to use commodity hardware because what they are routing works with a non-traditional setup...specifically a Clos network. That works for them only because their app is designed in a way that it can served up like that. It would not work for the typical kinds of traffic that you see at a normal company.

At a certain scale, barring unusual exceptions, you really do have to go with someone like Juniper/Cisco, as the ASIC based acceleration and other features aren't there in the commodity world.

There's certainly some things in progress that may change all of that, like Intel's DPDK, QuickAssist, etc.