Hi
As Leo stated there will be a hit but i was able to minimize mine doing it like this if it helps , there is no whitepaper on this just on moving from standalone to VSS that i have seen anyway , no VSS to VSS migration doc , I would do this in a shut down scenario its cleaner
To do this live too the FW would need a connection to the 9600 too , so when your migrating traffic doesnt just fall off when its moved to the 9600 side , the path upstream needs to be there too at same time on both clusters
1 create a replication of all layer 2 vlans and SVIs on the 9600 that are on the 6500 cluster keep them in shut down state , make sure its identical both setups
2 Create layer 2 trunk connection between both VSS clusters allow all vlans
3 Take 1 vlan as a test , get a pc on it at access layer ping right up through an past its GW so you know path is ok keep the ping running
4 Shutdown the SVI on the 6500 side and instantly bring it up on the 9600 cluster at same time , ping drop should only be like 1 or 2 if its done right.
5 Repeat the process for each SVI until all layer 3 interfaces have been moved over and the 9500 is the doing the routing , 6500 becomes a layer 2 VSS
6 Now you need to move the access layer link from cluster 6500 to cluster 9500 , what you can do to minimize it depends on spare ports , if the access switch has free ports ,create a secondary set port-channel but connect it to the 9600 cluster keep it in shutdown state and set the STP cost on each end to something really high 1000000 , make sure switches are in some form of rapid stp so the convergence is quick at layer 2
7 Once your happy with the config and STP is in place you can bring up the secondary PO it wont do anything due to the STP cost but as soon as you shut down say port-channel A it will bring up the only other valid weighted path STP port-channel
Thats off my head how i did it the last time , i would map out every task and possibility that could go wrong when doing this live before touching it , best approach if possible shut it down and move it over in a mntc window, a lot more can go wrong trying to do this live
