HA Agent not reaching isolation address

vSphere HA agent on this host could not reach the isolation address

A VMware High Availability (HA) cluster reports an error similar to:
Could not reach isolation address: 10.10.0.79
You will see an error because this is not a valid gateway address.
Regularly HA contacts (using ICMP ping) its isolation response address to ensure the cluster is functioning correctly. The default isolation response address is the default gateway but other reliable IP addresses can be configured along with the default gateway or the default gateway can also be disabled as an isolation response address.

1. In the vCenter user interface, in the Hosts and Clusters view, with the ESXi host selected, you see the message "vSphere ha agent on this host could not reach isolation address '6.0.0.0'".
2. 6.0.0.0 is configured as the gateway of DefaultTcpIpStack

To check this:

1. Connect to the vCenter Server through the vSphere Client
2. Select Hosts and Clusters, select the ESXi host object
3. Select Configure > Networking > TCP/IP configuration The default TCP/IP stack gateway is set to '6.0.0.0'

A new vSwitch and vmk port is created automatically when the Integrated Management Module (IMM) is connected.

If 0.0.0.1/24 is configured as the IP address by IMM, an ESXi bug will be triggered, and 6.0.0.0 will be set as the gateway of DefaultTcpIpStack,

This issue can occur when HA does not receive a response from all of its configured isolation response addresses.
Due to this the vSphere HA agent cannot be implemented on the affected ESXi host.
Changing network settings using these commands may make the host unusable and can necessitate a reboot to recover.
This issue is resolved in VMware ESXi 7.0 Update 1 (build number 16850804).

Steps

1. Modify the IMM configuration to make sure a legal IPv4 address is set, but not 0.0.0.1/24
2. Modify ESXi configuration to clear the notification "vSphere ha agent on this host could not reach isolation address '6.0.0.0'": a. Connect to the vCenter Server through the vSphere Client b. Select Hosts and Clusters, select the ESXi host object c. Select Configure > Networking > VMkernel adapters d. Select each vmk to check if the gateway is set to 6.0.0.0. e. With a vmk selected, In the pane below the adapter list select the All tab

If the display below under IPv4 settings shows the Default gateway is '6.0.0.0':
Either select the 'X' icon to delete the vmk, or
Select the pencil (edit) icon to edit the settings:

1. Select IPv4 settings
2. Select Use static IPv4 settings
3. Enable Override default gateway for this adapter
4. Set the gateway to a valid address

Check the management network gateway setting in the ESXi host Direct Console User Interface (DCUI)

a. Access to the ESXi host DCUI.

Note: This can be done by direct connection to the server hardware, back-end management system such as iDRAC, iLO, UCS, or from an SSH session.

b. Select IPv4 Configuration. If the gateway is set to '6.0.0.0' and use the Enter key to change settings

c. Set the default gateway to a valid IP address.

d. Restart management agents.

This issue can be reproduced by setting up ESXi with default network configuration, vmk0 and vSwitch0:

a. esxcli network vswitch standard portgroup add --portgroup-name="test-pg" --vswitch-name=vSwitch0
b. esxcli network vswitch standard portgroup list
c. esxcli network ip interface add --interface-name=vmk1 --portgroup-name=test-pg
d. esxcli network ip interface ipv4 set --interface-name=vmk1 --type=static --ipv4=0.0.0.1 --netmask=255.255.255.0

Ref: https://kb.vmware.com/s/article/76737