When using multiple vCenters in the same Single Sign on Domain (Enhanced Linked Mode), there is high potential of corruption of the domain if no offline snapshots are taken of all nodes before the changes. 

If a change must be reverted, all nodes of the Enhanced Linked Mode domain have to be restored back to this offline/consistent snapshot. All nodes must be reverted to the snapshots first, before powering any on.

This article provides common workflows where VMware recommends snapshots to be taken in a powered off state for all nodes in the domain (vCenter Servers and any external Platform Service Controllers).

Note: VMware also recommend to use File-Based or Image-Based backup For more information, see  Overview of Backup and Restore options in vCenter Server 6.x/7.0.x (2149237) .

Please also see, vCenter Server Appliance Data Integrity Best Practices (59230) .

For more information about ELM for vCenter 6.x, see vCenter Enhanced Linked Mode .

For more information about ELM for vCenter 7.x. see vCenter Enhanced Linked Mode .

VMware vCenter Server Appliance 6.5.x
VMware vCenter Server 7.0.x
VMware vCenter Server Appliance 6.7.x
VMware vCenter Server 6.x

VMware recommends to have an offline Snapshot  (virtual machine powered off) of all nodes in the same SSO domain "in ELM" before any activity that will include changes in the vCenter Server.

Offline snapshots of all nodes in an SSO domain (ELM) are required when any SSO domain update will be performed. This includes but is not limited to:

• vCenter Server Updates (Full Version, Update Release, or Patch Release).
• Using the lsdoctor tool to make any changes.
• Adding a new vCenter Server to an existing SSO domain.
• Retiring a vCenter Server from an existing SSO domain.
• Certificate Replacement (Machine, CA, STS, etc).

Any read only activity does not require offline snapshot. That means the below activities can be performed with NO need for snapshot (Neither online nor offline). 

• vCenter Server / PSC backups.
• Using lsdoctor tool  --lscheck parameter.
• Using the checksts script.

Caution:

• Offline snapshot is a MUST when using lsdoctor tool unless using the lscheck switch (only) which is used to check for common issues in the lookup service.
• Make sure there is no backup task running before taking the snapshot of the vCenter Server or the PSC.
• Do not take snapshot of the vCenter VM(s) if it’s part of vCenter High Availability (VCHA) configuration. 

For more information about vCenter update, see Patching (update) VMware vCenter Server Appliance VCSA and Platform Service Controller PSC through Appliance Management Interface VAMI (84322).

For more information about lsdoctor, see Using the 'lsdoctor' Tool (80469).

For more information about checksts script, see Checking Expiration of STS Certificate on vCenter Servers (79248).

For more information about joining vCenter to an existing SSO Domain, see Joining a vCenter Enhanced Linked Mode Domain.

For more about retiring vCenter Server from existing SSO Domain, see Using the cmsso command to unregister vCenter or PSC from Single Sign-On (2106736).

For more information about vCenter Security Certificates, see vSphere Security Certificates.

For more information about vCenter High Availability, see FAQ: vCenter High Availability (2148003)