Why OpenBSD? CARP - redundancy

Service redundancy was needed, so

carp (4) virtual network interface - patent free successor to Cisco's VRRP router redundancy protocol, for general redundancy. Introduced in OpenBSD 3.5 (May 1 2004)

Virtual IP address for failover or load balancing modes

With ifstated(8) helper daemon toolbox 'full service cluster solution'

Firewall redundancy with state table sync via pfsync(4) (again a virtual network interface)