Recovering Disabled and Out of Sync Nodes in an ISE deployment

Replication Disabled or Out of Sync:

Replication status indicates the health of communication of the sync between PAN and other nodes in a deployment.  Primary Admin nodes sync config changes and updates to all other nodes in a deployment in the form of replication messages.  For connectivity or communication issues, if a node goes out of sync or is unreachable, it will no longer receive or consume updates from the admin node and will need to be recovered.

Hovering over the Target next to the Node Status under Administration > Deployment will show you the current status of the node's replication and if it is in sync, how many messages are pending to be synced.  If replication has failed, you will see a message indicating you need to perform a manual sync if the node is still reachable but out of sync.

If the node is not reachable however, in instances where it may have powered off or in case of connectivity issues, it will indicate a different status.  Ultimately, the action to take is the same, ensure reachability and perform a manual resync.

To perform a manual sync:

Navigate to Administration > Deployment and select the Node that is in need of a manual synchronization with the checkbox next to it:

Performing a manual sync will involve a reload of the target node but not the Primary Admin node.

There might be situations where if the node has been out of sync for a long time, it may not be possible to recover via a manual sync. In these conditions, the quickest way to recover is to Deregister the node from the deployment, perform an application reset-config via CLI.  Once the config has been reset, you can register the node back to the deployment.