Database BRANCH is running at a remote site and communicates with Database HOST and HOSTDR (Two-Node Oracle RACs) via a database link. The TNS entry was recently change to a format intended to ensure that when a Dataguard Switch is performance between HOST and HOSTDR, it would not be necessary to update the TNS entry on BRANCH (Connect Time Failover)
Following the change we found where was an abundance of Virtual Circuit Waits on the BRANCH database. Sessions would wait up to 30 seconds on this wait event and user experience was poor. An AWR difference report showed clearly that this problem was related to the change.
After much troubleshooting, we used the VIPs of the HOST and HOSTDR clusters respectively rather than the SCAN IPs. Virtual Circuit Waits dissappeared.