vDS (Distributed Switch) Environment
If an ESX host is using a vDS some network settings, including VLAN settings, on the console are grayed out. This means you need to change the VLAN settings using vCenter. After you change a hosts management network via vCenter to tag packets in a particular VLAN it throws up an error and disconnects the host. But then it comes back online a few minutes later. This is because from vSphere 5.1 there is a feature called Network Rollback which is enabled by default. This feature is designed to stop us taking a host offline by making incorrect network changes and if vCenter loses connectivity with a host the network changes will be rolled back.
Let’s assume a port on a Cisco switch is configured as an access port in VLAN 100. We then change the ESX host to tag the traffic it sends with a VLAN 100 tag, this causes the port to drop these frames as it’s an access port. Because of this vCenter loses connectivity with the host and the change is rolled back. Even if we quickly change the Cisco port to a ‘trunk’ port the update will not stick.
You can disable the Network Rollback feature using the steps below.
1. Browse to your vCenter Server using a web browser (https://vcenter_IP:9443)
2. Navigate to the required vCenter server and click the ‘manage’ tab, then select ‘settings’
3. Go into ‘advanced settings’ and click on ‘edit’
4. Find the ‘config.vpxd.network.rollback’ key, and change the value from ‘true’ to ‘false’
5. If the key is not present, you can add it and set the value to false
6. Click OK.
7. If this didn’t work you might need to restart the vCenter services for the changes to take affect.
Once this is done you can once again make the change in vCenter. vCenter will lose access and the host will be marked as disconnected but once you update the Cisco port to a trunk the host will come back online.
vSwitch Environment
When using a standard vSwitch the ‘Network Adapter’ and ‘VLAN (optional)’ settings in the ESX console are not grayed out. This means you can change the VLAN information there and vCenter will pick it up once the port on the switch is changed to a trunk port.
