Hopefully one of the networking guys who are lurking here is going to correct me if I am wrong but I don't even think you need fully blown OS to control layer 3 switch like the one from your link. You can use SNMP and serial port to configure full manged switch like this. Sure enough a custom FreeBSD (JunoOS) is used for proprietary Juniper switches but not vanilla version.
I'm guessing I'm one of those networking guys.

I'll chime in below.
You either need switch or you don't. If you need a switch buy a switch. If you need a good network server I have two of these running OpenBSD
Instead on focusing on the OS I strongly suggest to focus on the switch. If a switch runs Linux, who cares? As long as it actually does a good job.
Deever, these guys are nailing it. First of all what is the problem you are trying to fix? How do you manage a dozen switches or so, or hundreds/thousands of switches; is that the problem? If you have only a few switches then stick to ssh, and/or snmp, and/or gui tools. Got a couple dozen then fine try using a nice management platform via snmp. And if you require a heavy duty rock solid switching platform then consider Juniper. I believe you will not be disappointed. These 'big iron' switches have specially made ASICs for the purpose of forwarding ethernet frames; it gets pricey for a kernal/cpu/motherboard to compete with that.
Back to finding switches running a Linux kernel, please keep in mind this is early days, and keep in mind that the 'initial market purpose' is not just for management, but for Software Defined Networking (SDN) purposes. That is where a really large diverse enterprise company, or a service provider company will deploy a SDN based network so that they can provision network services (including billing) over a freaken large/big network (often comprised of different vendors, different platforms with different CLI formats) in a matter of minutes, not days or weeks. The problem they are fixing is their "time to service delivery". Just to be clear you do not need a switch running a linux kernal to achieve that. Any big iron manufacture worth their salt is working to support SDN. Last but not least the CLI on the good quality switches (like Juniper) has been optimized for the user doing the CLI. I don't know if the 'Linux switches' have a well thought out CLI.
All of the above remains for the same regardless if your switch is a 'layer 2 switching' only device, or if it
also does 'layer 3 routing' implemented in switching ASICs.
But to correct Oko, if you need a good network server use FreeBSD.
