Several VLANs, same IP

[tomas.jacobsson]

Here's a weird one.

I have 3 industrial machines with the same hard coded IP address, 10.255.10.10/24. I need 3 different laptops to communicate with it's respective machine through a 2504 WLC. On the switch it's not a problem, I configure VLAN10, VLAN20 and VLAN30 for each machine without assigning any IP address to them. To create these VLAN interfaces on the WLC it's mandatory to provide a local IP address. Since all interfaces belong to the same scope this is impossible, and to get around it I have configured 3 dummy addresses for the interfaces; 2.2.2.2/30, 2.2.2.6/30 and 2.2.2.10/30. The laptops get static IPs 10.255.10.101, 10.255.10.102 and 10.255.10.103 and they can communicate without problems with their respective machine. 

The question is: Could I run into trouble using fake IPs on the WLC interfaces. What are the purposes with these IPs? 

Thanks

/Tomas

[Stephen Rodriguez · Accepted solution]

you could run into issues, if a client on wireless needed to access content at one of those addresses.

The reason you need to put an IP address on the WLC, is for roaming purposes. With out an IP address there, the WLC would not know if a Layer 3 roam or Layer 2 roam happened.

Example:

Buidling A

VLAN 10 10.10.10.x

VLAN 20 10.10.20.x

Building B

VLAN 10 10.20.10.x

VLAN 20 10.20.20.x

If you roam on the wireless between buildings, and the WLC only looked at the VLAN ID, your traffic would blackhole unless/until the client attempted to do DHCP, which isn't required to happen on a roam.

By putting an IP address on the interfaces on each WLC, the WLC would know that the client had made a layer 3 roam, and would then anchor traffic to the appropriate WLC so that traffic flow isn't interrupted.

HTH,
Steve

[tomas.jacobsson]

Thanks a lot for the answer.

/Tomas