Duplicate IP Dectected on two VLANs

[MT-849]

Hi,

We have an issue where people on VLAN 1 are starting to get an IP address from the DHCP scope on VLAN 6. The switches all look to be set up fine and they used to work with no issues.

One thing I have started to see a log of on our core switches are these messages:

02581 ip: ST2-CMDR: IPv4: Duplicate IPv4 address 10.10.15.4
            is detected on port 1/1 in VLAN 1 with a MAC address of
            645106-bb5aa3 (5 times in 60 seconds)
02581 ip: ST2-CMDR: IPv4: Duplicate IPv4 address
            10.10.11.253 is detected on port 2/2 in VLAN 6 with a MAC address of
            645106-bb5aa3 (6 times in 60 seconds)

That mac address is the mac of our core switches (2x Procurve 2920 stacked)
The IP 10.10.11.253 is the IP of our core switch and 10.10.15.4 is the IP of the core switch on that VLAN

          VLAN           IP Config     IP Address       Subnet Mask
  -------------------- + ----------  ---------------  ---------------
  DEFAULT_VLAN         | Manual      10.10.11.253     255.255.255.0
  Voice                | Manual      10.10.15.4       255.255.255.0

I know using VLAN 1/DEFAUL_VLAN isn't best practice and is on my ever growing list of things to change

Could this be caused by a a loop/bridge between the two VLANs somewhere or a config issue on the switches?

Cheers

[BjKo]

How is the link between your core stack and the edge switches configured? LACP and every VLAN tagged (even VLAN 1)?

[MT-849 · Accepted solution]

Hi,

I managed to find the issue over the weekend, someone had plugged a cable from VLAN6 into a device that was connected to VLAN1 already creating a bridge between the two

It would be good to see if there is a way to prevent that from happening again though

Cheers