pfSense » pfSense Docs

Page: https://docs.netgate.com/pfsense/en/latest/troubleshooting/ha-dhcp-failover.html

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Both nodes must have the same interface definitions such that the same DHCP scope is on the same OPT interface. For example, if you create a new interface pair 'DMZ' on both nodes sharing a VIP, but one is OPT3 and the other is OPT6, IP will work, VIP will work, but DHCP peering will fail and rules propagation will fail via pfsync. Ensure that both nodes have the same interface number for the same DHCP scope. Create dummy/placeholder interfaces (e.g. assign unused VLANs) on the other node if needed to get the new interface to the same OPT number.

Both nodes must be running the same version of pfSense® software. Update both nodes to the newest available stable release if they do not match. Older versions may have problems with various aspects of DHCP failover that have already been corrected.