Re-designing a layer 3 routed network to a layer 2 flat network

[clybumat1]

All has been going fine so far, but I've run into two user switches that are both using the same layer 3 vlan, vlan 80.  One switch has a 10.4.80.1/25 network, while the other has a 10.4.80.128/25 network

This works ok in the current environment because both switches are layer 3 switches, basically acting as two separate cores.  Well in the new design, every switch is changing to layer 2, and all layer 3 vlans will be setup on a single core. 

How can I setup both of these 80 vlans on the core?  You can't have two vlans of the same name.  How will each layer 2 switch know which 80 network to be on?  The PC's have static IP's configured, with /25 subnets (255.255.255.128)

[Milos Megis]

I cannot tell you how exactly do it because I don´t have experience with exact configuration but it sounds like good opportunity for implementation of QinQ (queue in queue).

[Joseph W. Doherty]

Milos, it's unclear to me how QinQ would help in this situation. Could you expand upon your thinking on how it would be used?  (NB: BTW, when you write QinQ I'm thinking of IEEE's 802.1ad, is that what you're referencing?)

[devils_advocate · Accepted solution]

Just change the VLAN ID, it will be the easier thing to do in the long run.

How many switchports can a /25 subnet have? 

The subnets don't overlap, just the VLAN ID's so change one of them.

Its hardly loads of work to change some switchports, use interface range or something like Solarwinds NCM if you have it. 

The PC's have no concept of what VLAN they are on so they can stay with the same IP's.

Find a maintenance window, create the new VLAN ID on the 'core' and assign its SVI the gateway address. Then login to each switch and change the ports to be in the new vlan. Bosh, done.

[clybumat1]

Great answer, devils_advocate.  So basically one layer 2 switch will stay on the vlan 80, the SVI on the core will be the 10.4.80.1/25, and the other switch will get a new vlan ID, perhaps vlan 800, and the SVI on the core will be the 10.4.80.128/25. 

Thanks again!

[devils_advocate]

Exactly, the subnets do not overlap, only the VLAN ID's do.

As I said, assuming these are normal hosts (PC's, Laptops, Printers etc) then they have no concept of the VLAN they are, they simply send frames into the switch and the switch knows what VLAN they are part of due to the Access VLAN configured on the port.

There will be downtime obviously but based on what you are doing, I suspect you have planned this already. 

Just make sure the new Vlan is allowed on the trunk links between the Core and the Access Switch (es).