Duplicate MAC addresses

Confirm by checking the physical MAC address of all physical NICs and their link status, the existing vSwitch configuration, and the current vmkernel interfaces configuration. For ESXi, use these commands:

# esxcfg-nics -l

If a duplicate MAC address is confirmed, the MAC address must be changed by deleting and recreating the vmkernel interface.

 

To delete a vmknic from a port group, use this command:

# esxcfg-vmknic -d -p pgName

or

# esxcfg-vmknic -d pgName

To add a vmknic to a port group, run the command:

# esxcfg-vmknic -a -i DHCP -p pgName

or

# esxcfg-vmknic -a -i x.x.x.x -n 255.255.255.0 pgName

 

 

To work around the issue, manually configure the MAC address on the ESXi host:

Additional Information

If the /Net/FollowHardwareMac option is not being honored after a host reboot on ESXi 7.x, ensure sure the mac_mode of vmk0 is set to PNIC_BASED in the ESXi host configstore.

The steps to check and properly update the configstore are:
1) Export current configstore vmknic settings:
# configstorecli config current get -c esx -g network -k vmknics >> /tmp/My-Net.json
2) Remove the existing "mac" line and modify the "mac_mode" and "nic" settings in the file for vmk0:
# vi /tmp/My-Net.json  <---- Delete the current "mac" line, then add/modify the two lines below: 
   "mac_mode": "PNIC_BASED",
   "nic": "vmnic0",
# cat /tmp/My-Net.json (verify)

Example: 
[root@vESXi70-2:~] cat /tmp/My-Net.json
[
  {
   "device": "vmk0",
   "enabled": true,
   "enabled_services": {
     "management": true
   },
   "ip": {
     "dhcp": false,
     "dhcp_dns": false,
     "ipv4_address": "10.0.10.204",
     "ipv4_subnet_mask": "255.255.255.0",
     "ipv6": {
      "auto_configuration_enabled": true,
      "dhcp": false,
      "dhcp_dns": false
     },
     "ipv6_enabled": true
   },
   "ip_route_default_gateway": "10.0.10.1",
   "ip_route_ip_v6_default_gateway": "::",
   "mac_mode": "PNIC_BASED", <------------------ modify this line  
   "nic": "vmnic0",              <------------------ added this line
   "net_stack_instance_key": "defaultTcpipStack",
   "port_connection_type": "VSS_PORT_GROUP",
   "port_group": "VMkernel",
   "tso_mss": 0
  }
]
3) Create a backup copy of the Config-store, just in case: 
# cp /tmp/My-Net.json /My-Net.json
4) Import the changes made to /tmp/My-Net.json into the configstore:
# configstorecli config current set -c esx -g network -k vmknics -infile /tmp/My-Net.json
5) Confirm the changes made to the configstore:
# configstorecli config current get -c esx -g network -k vmknics
 

 

Creating an Alarm to Monitor Duplicate IP address: This method can be applied to warn of duplicate MAC addresses as well.

In addition to logging to the VMkernel logs, ESXi also logs this observation in /var/log/vobd.log file which stands for the VMkernel Observation. These observations can provide critical identifying information in case of an error and is usually used during troubleshooting. In our case, if we are seeing an intermittent network connectivity to our ESXi host which is in result of a duplicate IP Address. Utilizing these VOBs is that you can create vCenter Alarms when a specific VOB has been detected.

You can do exactly the same for detecting a duplicate IP Address for an ESXi host. First, identify the VOB ID by looking in the /var/log/vobd.log file (for Duplicate IP address or Duplicate MAC address):

YYYY-01-21T15:02:07.513Z: [netCorrelator] 917174784727us: [esx.problem.net.vmknic.ip.duplicate] Duplicate IP address detected for xx.xx.xx.xx on interface vmk0, current owner being xx:xx:xx:xx:xx:xx

Note:

To create an alarm: