The migration would be 3 phase approach
1) Distribution/access layer relocation
2) Inter-vlan routing migration
3) Wan routing migration (only if nat isn’t running)
You would require 3 additional interconnects originating from the new core switch/stack which would include:
1x L2 trunk interconnection (into lan handoff switch)
1x L3 routed interface into existing ROS device (preferably p2p)
1x L3 routed interface for relocation of ROS ISP/WAN link



						Phase1
						------
On new core switch create the SVI’s to the same ip addressing of the exiting ROS but do not enabled them, also enable the dynamic routing process on them.
Create the trunks interconnects for distribution/access switches
Re-create any dhcp scopes that ROS it has running.

1) Distribution/access layer relocation
 Create the 1x L2 trunk interconnection (into lan handoff switch) and allow all vlans across this trunk, once attached to the lan handoff switch your new core switch with the help of vtp should populate itself with the existing L2 vlans running on the estate.

Relocate all distribution/access switch onto new core switch.




						Phase2
						------
On the new Core and ROS create 1x L3 routed interface with a /30 addressing 
(this interface will be temporary if you’re not running Nat)
Enable the dynamic routing on these interfaces.
At this point your new core switch should then receive the default route being advertise from the ISP/WAN device no service disruption should be incurred.

2) Inter-vlan routing migration
Disable one by one the sub-interfaces on the ROS device and enable its equal SVI on the New Core switch, (enable nat only on the ROS p2p interface)
At this point the routing for this vlan should convergence and users should begin to route via the new core ( any dhcp scope on the new core should also become active at this time)




						Phase3
						------
Once all inter-vlan routing has been migrated over to the new core then disable the lan interface on the ROS device
3) Wan routing migration (this can only be accomplished if you are not running NAT on the ROS)
Relocate the ROS ISP/WAN interconnect over to the new core into 1x L3 routed interface ISP/WAN link and once this interface becomes active lan users should be able route externally

Disable the 1x L3 p2p routed interface between the New core and ROS device
Disable the ROS wan interface and shutdown ROS device