High availability concepts

Let's introduce this topic with an aviation example. At the beginning of heavier-than-air history, airplanes had just one engine to fly from one location to another. As aviation grew, the demand for long-range flights increased, and new projects that used two or more engines began. Nowadays, it is possible to cross the oceans with a twin-engine plane thanks to reliable engine technology and the Extended-range Twin-engine Operations Performance Standards (ETOPS). But even with all this technology, two engines are required to keep a long-haul flight within safety standards. Developments similar to the aviation industry also happened in the IT world – redundancy standards/protocols were created to keep the availability of the systems at acceptable levels for the business.

In OPNsense, the Common Address Redundancy Protocol (CARP) is a protocol that ensures that the network interfaces of two or more firewalls keep operating in case of a hardware...