If there is no Multi Chassis Synchronization (MCS) between the two peer nodes, the two geo-redundant nodes are configured as two distinct realm nodes from the PCRF point of view. Each node acts independently of the other node. After a switch-over, AA on the newly active node detects new traffic flows with new IP addresses. AA notifies PCRF with a CCR-I message to retrieve subscriber policies. After PCRF confirms the same IP address used on a different Gx session ID, it deletes the old session for that IP address.
Similar behavior takes place if an MCS is configured with session IDs and OSI states journaled across, as per ESM implementation. The two geo-redundant nodes are configured with the same host realm, and appear to PCRF as one node. After a switch-over, AA-ISA on the newly active node detects new traffic flows with new IP addresses. AA-ISA notifies PCRF with a CCR-I message to retrieve subscriber policies. The Gx session ID used is unique and different from the session ID used on the previously active node. After PCRF confirms the same IP address used on a different Gx session ID, it deletes the old session for that IP address.