I assume that you already know what Lockdown Mode is. With Lockdown Mode enabled ESXi server, you will need to use vCenter to Administrator ESXi Server therefore you can enforce centrally managed permission control via vCenter Server, rather than configuring and managing permissions at each ESXi Server.
When Lockdown Mode is configured on ESXi server, vCenter Server has to be highly available as Loss of vCenter Server makes your ESXi Server unmanageable. Luckily you have a workaround. if your vCneter Server goes down, and need to access ESXi Server, then you can login to DCUI (Direct Console User Interface) of ESXi Server to disable Lockdown Mode.
The steps to enable Lockdown Mode and disable Lockdown Mode from DUCI
1. Enable Lockdown Mode from vCenter Server
You can enable Lockdown Mode from Host > Configuration > Security Profile > Lockdown Mode
2. Verify Lockdown Mode operation. You will get No Permission Error.
3. Disable Lockdown Mode via DCUI.
Login to DCUI then select “Configure Lockdown Mode” to disable Lockdown Mode from DCUI.
4. Verify the direct access to ESXi Server, you should be able to login now.
This workaround is all good, but it leave a security hole. If you know this workaround, then you can always bypass vCenter Server then login to ESXi Server directly with potential Administrator access
Total Lockdown Mode will prohibits this workaround by disabling DCUI. If you disable DCUI then you cannot login to DCUI thus you cannot disable Lockdown Mode. This will enforce the use of vCenter Server. With Total Lockdown Mode, the availability of vCenter Server is crucial.
Configuring Total Lockdown Mode
1. Make sure that Lockdown Mode is enabled.
2. Stop DCUI Service from Host > Configuration > Security Profile > Services > Properties
3. Select Direct Console UI. Then Options.
Click Stop to stop DCUI.
4. If you try to login through DCUI, you will receive the error message.
VMware Blog:
http://blogs.vmware.com/esxi/2010/09/the-new-lockdown-mode-in-esxi-41.html