The original formulation of the question was fairly straightforward: there is a site doing inter vlan routing using a 4331 doing router on a stick with a 9300 switch. The switch is configured with vlans and trunks the vlans to the 4331, where routing takes place. The objective is to transition to an environment where the 9300 does the intervlan routing. I believe that this is possible.
The question does not tell us what will be the role of the 4331 when this transition takes place. Will the 4331 still be in the network to route to things that are not locally connected? Or will the routing to remote destinations be done on the 9300?
I would suggest these steps to make the transition:
1) this assumes that the 9300 has a management vlan interface that has IP connectivity to the 4331. If this is not the case then setting it up is the first step in the transition.
2) enable IP routing on the 9300.
3) configure a static default route on the 9300 with the 4331 as the next hop using the IP that had been specified in the default-gateway command.
4) for each of the vlans on the 9300 do these steps
** configure the vlan interface.
** on the vlan interface configure an IP address that is in the subnet but that is different from the IP used by the 4331.
** configure other appropriate interface parameters to match what might have been configured on the 4331.
5) as the vlan interfaces are configured the 9300 will be capable of routing between those vlans (in addition to the routing performed by the 4331).
6) when all the vlan interfaces are configured and operational you can begin to transition away from the 4331 for inter vlan routing.
7) on each vlan interface on the 9300 configure a secondary IP address that uses the IP address configured in that subnet on the 4331. (it will result in 2 devices claiming to have the same IP address, but I do not believe it would be a problem - and it will not last long)
when the secondary address has been configured on the 9300 then remove the IP address from the subinterface on the 4331. This will result in all of the inter vlan routing for that subnet being done on the 9300. And anything trying to access the IP that had been on the 4331 will find it on the 9300 secondary address.
9) in each vlan interface on the 9300 remove the IP address configured in step 4. This will result in the "secondary" address becoming the primary address (indeed the only IP address) for the interface.
10) after these steps it probably makes sense to remove the subinterfaces configured on the 4331 since they no longer serve any purpose.
I believe that these steps should successfully transition from the inter vlan routing on 4331 to 9300 with no (or at least minimal) interruption. It does not address the question of what to do about the static default route sending traffic to the 4331. Resolving the question of what ongoing role the 4331 will play may also help answer the additional aspect that emerged which was OSPF on the 4331. It seems to me that in step 4 part of the other interface parameters might include setting up OSPF and running it on each vlan interface.
HTH
Rick
