| 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 :
- 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
- 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
- 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. |
|
|
|
|