Ensure all DNS forward/reverse resolution is successful between the Platform Services Controllers, vCenter Servers, ESXi hosts and the virtual machines involved in your upgrade.
Ensure that NTP is configured correctly across all nodes involved in your vSphere upgrade.
Before upgrading, powered-off concurrent snapshots are recommended for all vCenter Servers and Platform Services Controllers within the Single Sign-On domain to prevent replication sync issues
If a rollback to snapshot is necessary, all PSC and vCenter Servers should be reverted to the previous state.
When upgrading several vCenter Servers with external Platform Services Controllers, replication partnerships are specified between the newly embedded vCenters during the upgrade configuration. These agreements should be form a consecutive ring topology. In the case of 3 or more vCenter Servers, the very last agreement is created between the first and last updated vCenter with the vdcrepadmin createagreement command.