Private IP Address Routes to L3 Subnet on Dual vNIC VM

It’s not easy for me to describe the issue in one line on the title. Let me give some background here. I have 2 set of VMs. Set 1 has VM A & VM B. Set 2 has VM C & VM D. Each VM has a vNIC configured with a private IP address. VM A and VM C also have another vNIC configured with an L3 (Routable) IP address. Each set’s private IP addresses are the same. To make sure no confusion I implemented a vRouter VM for each set. The vRouter is same as VM A or VM C, it has two vNICs. One is connected to L3 network, another is connected to the private network. This way can keep the private network traffic not going outside of the set. So the both set no disturb each other when I set same private IP addresses.

Diagram

Following are IP addresses I set for each VM:

  • VM A: 192.168.0.11
  • VM B: 192.168.0.12
  • VM C: 192.168.0.11
  • VM D: 192.168.0.12

The problem is I still can get ping responding on VM A to 192.168.0.12 when I turn off VM B. I expected to see the L2 traffic goes to it own vRouter and finds VM B is offline. But tracert command shows me the traffic goes from VM A’s L3 network to vRouter of the 2nd set, and then get the answer from VM D. Looks like the L2 ping package is broadcasting on L3 network.

The issue was fixed by enabling a feature on L3 network. It called “Enforce Subnet Check for IP Learning“. Cisco changed the name to “Limit IP Learning To Subnet“. It’s a VLAN level setting. It will not allow broadcasting the private Ip traffic on an L3 network. It forces private IP traffic to go to L2 network only.

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