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.

The logical redundant interface is only available on ASA platforms and not on devices running FirePower.

Links:

https://www.cisco.com/c/en/us/td/docs/security/asa/asa97/configuration/general/asa-97-general-config/interface-echannel.html

https://community.cisco.com/t5/network-security/asa-firepower-failover-and-redundant-interface/td-p/4432054