I am trying to find out the best method/approach for changing out our core routers. We run 2 3550 switches in a collapsed core model that are running HSRP and being used as our core routers. I am installing a stack of 4 3750G switches and would like to make this stack our core routers and decommision our existing 3550's. What would be the best approach for this, taking in mind that the virtual IP used by HSRP needs to be in play on the new stack as all our campus switches point to that IP as their gateway. We run 25 VLAN's that are being routed by the current 3550's.