9.1 Dial Backup
9.1.1 Configuring dial backup
A backup interface is an interface that stays idle until certain circumstances occur, and then it is activated. The backup interface can be a physical interface or an assigned backup interface to be used in a dialer pool. Backup interface examples for a primary line can be an Integrated Services Digital Network (ISDN), an asynchronous interface, a dialer pool, or another serial interface. The backup interface is referred to often in Cisco documentation as the secondary link.

A backup interface can be configured to activate when the following situations occur:

  • The primary line goes down
  • The primary line reaches a certain load threshold

Backup interfaces are beneficial for redundancy, in case primary lines fail. The example in Figure illustrates an ISDN backup for a Frame Relay network.

To configure backup if a primary line goes down, perform the following steps :

  1. Select the primary interface, and configure it as needed (for dial-on-demand routing [DDR], Frame Relay interfaces and subinterfaces, X.25, and so on):

Router(config)#interface serial 0

  1. Use the following command on the primary interface to specify the interface or dialer interface to use for backup. (Interface number specifications vary from router to router. For example, some routers require you to just specify the port number; others require you to specify the slot and port.)

Router(config-if)#backup interface interface-type number

  1. Define the period of time to wait before enabling the backup link after the primary link goes down with the following command syntax, which is explained in Table :

Router(config-if)#backup delay {enable-delay | never} {disable-delay | never}

Lab Activity    
  You have just installed an ISDN line to be used as a dial backup line to your Frame Relay connection between the Atlanta (hub) and Orlando (stub) office. Configure DDR on the BRI interface of the Orlando router. Configure Frame Relay on the serial interfaces connecting to the Frame Relay network. Use floating static routes to control which link is used.