Hello,
the 'backup interface' command was very useful indeed.
As an alternative, you could configure two EEM scripts. In the example below, GigabitEthernet0/0 is the main, and GigabitEthernet0/1 the backup interface. If the main interface goes down, the script unshuts the backup interface, and vice versa:
event manager applet BACKUP_PORT_UP
event syslog pattern " %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to down"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface GigabitEthernet0/0"
action 4.0 cli command "shut"
action 5.0 cli command "exit"
action 6.0 cli command "interface GigabitEthernet0/1"
action 7.0 cli command "no shut"
action 8.0 cli command "exit"
event manager applet BACKUP_PORT_DOWN
event syslog pattern " %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/0, changed state to up"
action 1.0 cli command "enable"
action 2.0 cli command "conf t"
action 3.0 cli command "interface GigabitEthernet0/1"
action 4.0 cli command "shut"
action 5.0 cli command "exit"
action 6.0 cli command "interface GigabitEthernet0/"
action 7.0 cli command "no shut"
action 8.0 cli command "exit"