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High Availability

pfSense® software is capable of having multiple nodes act as a cluster for High Availability. This article is a brief overview. The Hardware Redundancy chapter in the pfSense Book should be consulted before configuring a high availability cluster utilizing CARP.

Settings for High Availability are found under System > High Avail. Sync, and sometimes on other areas that have special handling (Vouchers, packages, etc.).

Often such collections of hosts are referred to as “CARP Clusters” but CARP is only one aspect. High Availability is achieved using a combination of multiple related techniques, including CARP, State Synchronization (pfsync), and Configuration Synchronization (XMLRPC Sync).


Common Address Redundancy Protocol (CARP) is used by multiple nodes to “share” a Virtual IP address between multiple nodes in such a way that if the preferred node fails, another will take over seamlessly. CARP was originated by OpenBSD as an Open Source Alternative to the patent-encumbered VRRP.

CARP utilizes multicast, so care must be taken that the switches properly handle and do not block, filter, or limit multicast.

State Synchronization

State Synchronization is handled by pfsync which transmits state table information (contents, insertions, deletions, etc) to other nodes on a private/shared interface so that connection states are valid across all nodes. This way if a node fails and another takes over, user connections continue to flow and clients do not need to reconnect.

Configuration Synchronization

Configuration Synchronization is handled by XMLRPC Sync which is a mechanism of pfSense software that allows transmitting certain configuration information and commands between nodes. Typically the primary node will synchronize its configuration areas to a secondary node.

Common Requirements

For a cluster to function, a few things are required, such as:

  • Minimum of three IP addresses per subnet (one for primary, one for secondary, one or more for CARP VIPs) – This can be avoided on pfSense software version 2.2, but is still recommended.

  • A dedicated interface for state and configuration synchronization. For security reasons it is best to keep this isolated, though it could be run on a local network interface it is not advised.

  • Layer 2 equipment that properly handles multicast

  • Upstream/ISP/other involved routers that properly respect the addresses used by CARP.

Bringing it All Together

pfSense software makes it easy to set these areas up to create a High Availability cluster. The full details are available in the book linked above. A brief run-through of a basic CARP configuration may be found at: Configuring pfSense Hardware Redundancy (CARP)

For other aspects of High Availability and CARP, see the links below to the related categories.