The fundamental principle, and the foundation of network HA, is network link redundancy and redundant hardware (network elements).
The underlying design principle is that, for a critical service, at least two equivalent systems should be provided and topologies chosen in a way that there always exist, at the least, two redundant paths to the next device. This is why for so many years many robust and scalable photonic network approaches have been based successfully on protected ring topologies (for example, Synchronous Digital Hierarchy/Synchronous Optical Network [SDH/SONET]) and Resilient Packet Ring (RPR). Just because a lot of folks disliked Token Ring technology for no apparent reason does not mean that ring topologies per se are inferior to bus architectures or star topologies; on the contrary. With a small number of network elements, point-to-point links will suffice. Usually a collapsed network core consists of three or four network elements (as shown in Figure 12-1).
My approach to network redundancy is that more than one alternative link is unnecessary and unjustifiable commercially.
Another concept is the provisioning of cold- and hot-standby equipment, meaning components that need power up and hardware configuration (versus up-and-running failover candidates). Occasionally, engineers or management throw hardware resources at a simple design problem. However, HA concepts that are too exhaustive add considerable complexity to networks, occasionally defeating the purpose (and at unjustifiable expense).
As an introduction to the challenge of HA, Figure 12-2 presents a typical corporate Internet connectivity example in two variants.