patent on HSRP. Nonetheless, IETF continued work on VRRP. After some debate, the IETF VRRP working group decided to approve the standard, despite its reliance on patented techniques, as long as Cisco made the patent available to third parties under reasonable and non-discriminatory licensing terms. Because VRRP fixed problems with the HSRP protocol, Cisco began using VRRP instead, while still claiming it as its own. Cisco informed the OpenBSD developers that it would enforce its patent on HSRP. Cisco's position may have been due to their lawsuit with Alcatel. As Cisco's licensing terms prevented an open-source VRRP implementation, the OpenBSD developers begun developing CARP instead. OpenBSD focuses on security. They designed CARP to use cryptography. This made CARP fundamentally different from VRRP and ensured that CARP did not infringe on Cisco's patent. CARP became available in October 2003. Later, it was integrated into FreeBSD (first released in May 2005 with FreeBSD 5.4), NetBSD and Linux (ucarp). While Cisco's US patent expired in 2014, the two incompatible protocols continue to coexist. Incompatibility with IETF standards OpenBSD uses VRRP's protocol number and MAC addresses. The OpenBSD project requested unique numbers from the Internet Assigned Numbers Authority (IANA) but was denied. To allocate numbers, IANA has several requirements. At the time, these were specified in RFC 2780. Requirements include participating in a collaborative, lengthy discussion process within the IETF and producing a detailed textual specification of the protocol. The OpenBSD developers met neither requirement. OpenBSD's website states the following: IANA had assigned protocol number 112 to VRRP (in 1998, via RFC 2338). Protocol number 112 remains in use by VRRP. CARP also uses a range of Ethernet MAC addresses which IEEE had assigned to IANA/IETF for the VRRP protocol. In spite of the overlap, it is still possible to use VRRP and CARP in the same broadcast domain, as long as the VRRP group ID and the CARP virtual host ID are different. See also Gateway Load Balancing Protocol (GLBP) HSRP pfsync VRRP IP network multipathing (IPMP) References External links UCARP: userland CARP implementation NetBSD port of CARP The OpenBSD song 3.5: "CARP License" and "Redundancy must be free" High-availability cluster computing OpenBSD FreeBSD First-hop redundancy protocols