Question : Migrate a Cisco Router-On-A-Stick type of network

I have a router-on-a-stick type configuration I inherited. I would like to move all the routing onto a 3550 hanging off the router and just use the router as a gateway. The problem I am facing is that everything on the network is in the default vlan. To make matters worse, we have a VPN router buried in the LAN and we also have radio shots coming from other sites into the LAN. Furthermore, we have multiple networks all running on the default vlan 1; to include, switch management, user IPs, servers, and printers. It’s a nightmare of a network. I am having a tough time trying to figure out how I can migrate the services onto the 3550 with minimal downtime. Any suggestions on how to best get this done?

Router config:

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 Interface FastEthernet0/0
Description LAN
Ip address x.x.28.x 255.255.255.0 secondary (user/servers)
Ip address x.x.155.x 255.255.255.0 secondary (LAN Management)
Ip address x.x.153.x 255.255.255.0 secondary (off-sites)
Ip address x.x.152.x 255.255.255.0 secondary (off-sites)
Ip address x.x.154.x 255.255.255.0 secondary (off-sites)
Ip address x.x.152.x 255.255.255.0 secondary (off-sites)
Ip address x.x.29.x 255.255.255.0 (users/servers)

Interface FastEthernet0/1
Description WAN


Ip route 0.0.0.0 0.0.0.0 “WAN”
Ip route x.x.156.x 255.255.255.0 x.x.153.253
Ip route x.x.157.x 255.255.255.0 x.x.153.253
Ip route x.x.158.x 255.255.255.0 FastEthernet0/0
Ip route x.x.159.x 255.255.255.0 x.x.153.249

Switch:

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It is a dumb switch configuration. The router is connected to a non-routed FastEthernet port.

interface Vlan1
 ip address x.x.155.211 255.255.255.0
 no ip route-cache

interface FastEthernet0/8
 description Possible Router Interface
 switchport mode dynamic desirable
 duplex full

Answer : Migrate a Cisco Router-On-A-Stick type of network

I just re-read your configuration, seems you currently have different subnet on VLAN1... Actually, I don't have tried as your configuration, if you want to make it standard, you can consider my setting above. If you just want to minimal the downtime, you can assign two ports on the layer 3 switch as a router port by "no switchport" command, then one is for those internal lan ip address, another one is for WAN. I think it will works too, but not recommend.