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Make sure you have the latest HA Module and FreePBX framework installed first. You can do that via Module Admin in the GUI or via the CLI.
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Since the Cluster is based on the FreePBX Distro 10.13.66 or Older, you should always follow the Upgrade Scripts for the FreePBX Distro here. Upgrade Scripts should be run on the Master Server first and, once completed, put the Master Server into standby and upgrade the new Master Server.
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Since upgrading FreePBX modules will upgrade files in /var/www/html/ and write dialplan in /etc/asterisk, all module upgrades will be synced across your cluster. Both of those directories are part of the cluster sync as outlined here.
What happens if I upgrade system RPMs on my cluster?
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