two vlans with same subnet

[Wassim Aouadi]

I'd like to make SW1 manageable. The situation is that I have the same subnet used on two different vlans: vlan1 (on SW1) and vlan1900 (on the rest of the network).

Suppose I want to keep the actual configuration on SW1 and add an ip address under vlan1 on CoreSW. Then, on CoreSW, I'd have two SVI with the same subnet.

Can you tell me what the impact will be, in terms of switching and routing? and do you suggest another solution?

thanks

[Edwin Summers]

I suspect when you attempt to do this, coresw will throw a message similar to "overlapping network with vlan1900" and refuse to accept the IP on vlan1.

As far as other solutions, "it depends".  What exactly are you trying to do and what is the purpose of each vlan?

Best,

Ed

[Ven Taylor]

It depends on when you do it too.

If you configure each L3 interface, then make your physical connections, you'll have your devices yelling that they have overlapping subnets and where.

Ven

[Wassim Aouadi]

Edwin, vlan 1900 is the management vlan across the network. There has been an exception on SW1, where we "forgot" to change the management vlan from 1 to 1900.

Ok, here's the issue in CLI terms. On CoreSW, I want to do the following:

interface vlan1

ip add 10.100.254.2 255.255.255.0

So the switch will have two SVI (1, 1900) in the same subnet (10.100.254.0). Will it be able to communicate with SW1?

[cadet alain]

Hi,

the switch won't let you do that, it will tell you you have overlapping subnets and will refuse the IP address on this SVI.

But you can use secondary addresses as a temporary fix so you can try :

int vlan 1

ip address 10.100.254.2 255.255.0 secondary

ip address x.x.x.x x.x.x.x  which will be primary IP in a non overlapping subnet

Regards.

Alain

[Edwin Summers]

wass.aouadi-

Looks like Cadet Alain has the other question answered, but your last post raised a follow-up question.  Is there a reason you do not add VLAN 1900 to SW1 and make it consistent with the rest of your network.  I would understand if your network cannot tolerate the downtime or something similar.  Based on your diagram, I see you're trunking all VLANs across the network, so I would assume that VLAN 1900 is not in use on SW1.  Just curious.

Best of luck,

Ed