Principle of Redundancy
A group of hosts using CARP is called a "group of redundancy". The group of redundancy allocates itself an IP address which is shared or divided among the members of the group. Within this group, a host is designated as "Master". The other members are called "slaves". The main host is that which "takes" the IP address. It answers any traffic or ARP request brought to the attention of this address. Each host can belong to several groups of redundancy. Each host must have a second unique IP address.
A common use of CARP is the creation of a group of redundant firewalls. The virtual IP address allotted to the group of redundancy is indicated as the address of the default router on the computers behind this group of firewalls. If the main firewall breaks down or is disconnected from the network, the virtual IP address will be taken by one of the firewall slaves and the service availability will not be interrupted.
Read more about this topic: Common Address Redundancy Protocol
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