Description:
Allow a router to put an interface in standby until the primary interface will have a link down event.
Only useful in point to point interfaces, it didn't works on ethernet links if you have a switch in the middle, and it also didn´t work on a point to multipoint interface.
Used frequently on ISDN to backup interfaces.
Configuration:
r(config-if)# backup interface fastethernetX/X
Example:
R1:
interface Ethernet0/0
ip address 192.168.2.1 255.255.255.0
interface Serial1/1
backup interface Ethernet0/0
ip address 192.168.1.1 255.255.255.0
ip route 1.1.1.1 255.255.255.255 192.168.1.2
ip route 1.1.1.1 255.255.255.255 192.168.2.2
R3:
interface Loopback1
ip address 1.1.1.1 255.255.255.255
interface Ethernet0/0
ip address 192.168.2.2 255.255.255.0
interface Serial1/2
ip address 192.168.1.2 255.255.255.0
clock rate 56000
Example of a ping shutting down active interface.
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!...............*Mar 1 00:23:53.203: %LINEPROTO-5-UPDOWN: Line protocol on Interface Serial1/1, changed state to down*Mar 1 00:23:53.211: BACKUP(Serial1/1): event = primary interface went down*Mar 1 00:23:53.215: BACKUP(Serial1/1): changed state to "waiting to backup"*Mar 1 00:23:53.231: BACKUP(Serial1/1): event = timer expired on primary*Mar 1 00:23:53.239: BACKUP(Serial1/1): secondary interface (Ethernet0/0) made active*Mar 1 00:23:53.243: BACKUP(Serial1/1): changed state to "backup mode".*Mar 1 00:23:55.231: %LINK-3-UPDOWN: Interface Ethernet0/0, changed state to up*Mar 1 00:23:56.231: %LINEPROTO-5-UPDOWN: Line protocol on Interface Ethernet0/0, changed state to up.!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!*Mar 1 00:23:56.235: BACKUP(Ethernet0/0): event = secondary interface came up!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
