F5 HA KEY CONCEPTS

High availability
(HA) makes sure that the server pool is ready for user requests in situations when your primary load balancer is down and you can redirect that traffic to your backup/secondary load balancer with very minimal downtime which is not noticeable by users.

Redundant devices
A redundant system is a type of BIG-IP system configuration that allows traffic processing to continue if a BIG-IP system becomes unavailable. A BIG-IP redundant system consists of two identically configured BIG-IP units. When an event occurs that prevents one of the BIG-IP units from processing network traffic, the peer unit in the redundant system immediately begins processing that traffic, and users experience no interruption in service.

Failover
Failover is a process that occurs when one system in a redundant system becomes unavailable, thereby causing the peer unit to assume the processing of traffic originally targeted for the unavailable unit. An essential element to making failover successful is a feature called ConfigSync which is a process where you replicate one unit’s main configuration file on the peer unit.

Device Trust domains
To provide failover or configuration sync, BIG-IP systems on the network must be in the same trust domain. The trust relationship between BIG-IP devices on the network is established through certificate-based authentication. BIG-IP devices in a trust domain can synchronize and failover their BIG-IP configuration data, and exchange status messages continuously.

Device groups
A device group is a collection of BIG-IP systems that have established a device trust and share data with each other. There are two device groups types:
sync-only and sync-failover.
A sync-only device group synchronizes only configuration data, such as policy data, but it does not synchronize failover objects. 
A sync-failover device group synchronizes configuration data and traffic group data for failover purposes. Use this configuration to fully synchronize two BIG-IP systems.

Traffic groups
A traffic group is a collection of related configuration objects that run on a BIG-IP system. Together, these objects process a particular type of traffic