ASA redundant interface

A logical redundant interface consists of a pair of physical interfaces: an active and a standby interface. When the active interface fails, the standby interface becomes active and starts passing traffic. You can configure a redundant interface to increase the ASA reliability. This feature is separate from device-level failover, but you can configure redundant interfaces as well as device-level failover if desired.

It is preferable to use other high availability features instead of redundant interfaces, such as Active/Standby and Active/Active failover, and by using a pair of switches that are stacked, use VSS, or Nexus devices that use vPC to name a few.