According to Wikipedia, High Availability is "a system design protocol and associated implementation that ensures a certain absolute degree of operational continuity during a given measurement period."

 HA (High Availability) in an network can be achieved through:
- redundancy
- failover
- fault tolerance

Every one of those must be implemented in the network.

Types of redundancy:
- Multiple Spaning Tree
- Nics Dual Homing
- Routing Protocol Load Balancing

Other redundancy types, available on CISCO devices:
- RPR - Route Processor Redundancy