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Common address redundancy protocol

(also CARP)

Common address redundancy protocol definition

The common address redundancy protocol (CARP) refers to a network protocol that allows multiple hosts on the same local network to share a set of IP addresses. Its primary purpose is to provide availability, especially if having a single point of failure is unacceptable.

See also: broadband router

How CARP works

In CARP, you have several network devices, like routers or firewalls, sharing a set of IP addresses, but only one called the master, is in charge at any given time. Other devices are not actively managing the network traffic, but act as backup services, prepared to take over if the master one can't do its job anymore.

To ensure a swift change, the master device sends regular signals notifying other devices that it’s available. If these notifications stop, one of the backups becomes the new master and starts managing the network. For network users, the change is invisible, allowing them to keep using the service without interruption.

Where is CARP used?

  • Web server load balancing. It is used to distribute the load among multiple web servers.
  • High firewall availability. In cybersecurity, firewall availability is critical. CARP ensures that if the primary firewall fails, a backup can take over seamlessly.
  • Email. It helps switch between multiple email servers and avoid disruptions.