You can configure backup relations between a pair of any Layer 2 Serial interfaces. A backup interface is activated when the primary interface fails. The backup interface is deactivated when the primary interface is restored. A PPP session, frame relay interface, frame relay sub-interface, Dialer interface, FastEthernet interface, or Loopback interface can serve as a backup interface to any other Serial interface on the same module including interfaces on different serial ports.
Note:
A frame relay interface in a primary or backup role overrides the role of its sub-interfaces.
If the FastEthernet interface serving as a backup interface is configured as a DHCP client, it sends no DHCP packets. Therefore, its IP address is not renewed until it becomes the primary interface. If the FastEthernet interface serving as a primary interface is configured as a DHCP client, the expiry of the leases on its IP address or no reception of an IP address does not activate the backup interface.
