Hi Marc,
Thanks for your response. Yes - it's always the 4 hubs that are responsible for authentication. Apart from the following recurring errors in the errors log, there aren't any other signs of stress on the system - CPU and memory look OK; nothing unexpected in /var/log/messages:
[24/Dec/2019:07:55:36.298853461 -0800] - ERR - NSMMReplicationPlugin - bind_and_check_pwp - agmt="cn=hub-<node name>" (<node name>:636) - Replication bind with SIMPLE auth failed: LDAP error -5 (Timed out) () [24/Dec/2019:07:55:39.323604233 -0800] - INFO - NSMMReplicationPlugin - bind_and_check_pwp - agmt="cn=hub-<node name>" (<node name>:636): Replication bind with SIMPLE auth resumed
Thanks, Trev