Is this possible with a static route?

[Robert_F]

I have an ISP dsl/router on 10.1.1.x subnet providing internet to a WAN port on a Gargoyle(OpenWRT) router. All LAN hosts are connected to the Gargoyle router on subnet 192.168.1.x.

I need to migrate the 192.168.1.x ip range to the 10.1.1.x range but need to do this over time on a production network.

Therefore I wonder if a static route on the Gargoyle router would allow the changing of a host's static IP from say 192.168.1.82 to 10.1.1.82 without the host losing access to the remaining 192.168.1.x hosts that haven't had there IPs changed yet.

Since the ISP router is already set to 10.1.1.x subnet is there a static route that I can add to the Gargoyle router that would enable the ISP router to be used for 10.1.1.x routing whilst migrating?

 

[jsmithepa]

You are doing double NAT. You can add a switch to the 10.1.1.x subnet and move your devices there, one by one. But you will notice the 10 subnet can't talk to the 192 subnet and vicerversa. if the subnets don't need to talk to each other move at will, otherwise I believe you have to do a static routing at the ISP box.

 

[bill001g]

Not easily. You are using the same subnet on 2 different interfaces on you gargoyle router. The problem is not so much the router it is the end pc. They will for example want to use the gateway say its 10.1.1.1. That is on wan side of your router but the pc will not know that and issue a ARP. Since ARP never leaves the lan segment it will never get a response.

Now if you mean you moving the pc to ISP router as you change the IP that to a point will work. You have to make sure the gargoyle does not have NAT for the 192 ip block. Now the traffic still needs to go to the wan interface of the gargoyle and your ISP router will need a static route if it supports it or you will need to put a static route in each end pc pointing to the wan ip.

The more common way to do this is assign secondary ip on the lan and overlap both subnets. Not sure if that will work since you still can not have the same subnet on the wan and lan. If you can change the ip block between the 2 routers to something else until you are ready to cut that will work.

 

[Robert_F]

jsmithepa:
I'm avoiding double NAT by putting the Gargoyle Router into the ISP Router's DMZ. The subnets need to talk to each other. ISP box doesn't allow static routing.


bill001g:
If I need to drop the 10.1.1.x from the ISP router that is not a problem as nothing is plugged into it apart from the Gargoyle router for internet purposes. I hoped to use the ISP Router's routing abilities if possible which, now that I understand the ARP problem you describe, sounds like a no go.

Since the Gargoyle router is providing QoS, VPN and other services I am reluctant to disconnect hosts from the Gargoyle router unless I can retain it's services after doing so.

The more common approach that you mention sounds good. As mentioned I can change the subnet on the ISP Router and relocate the 10.1.1.x subnet. The issue therefore is how do I now create a secondary ip and overlapping subnet?

 

[bill001g]

At the moment I forget how exactly you do it on a gargoyle. In most device like this there is a field on the interface config that lets you key in a secondary ip and mask. Even windows pc have this ability.

Be aware you can only run DHCP on the primary ip in most devices....It is possible with helpers and static dhcp assignments to run overlapping but it is messy.

 

[Robert_F]

I've managed to add a secondary ip address to the Gargoyle routers LAN interface via command line. Works like a charm and the two ip ranges are now working in parallel with accessibility both ways.

Thanks bill001g!