Do you need specific software to make it work?
Should it reside in routers or in Ethernet LAN switches?
It will depend on whether you are using H.323, SIP or DEC for your VoIP, but what happens is that the routers and layer 3 switches look at the quality of service bit and determines if the packets should be processed ahead of other packets. It will throttle back data (which can stand a few re-transmissions) and let voice go ahead (which does not do well with re-transmissions.)
This was first published in March 2006