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This chapter describes how to configure the primary interface to use the dial backup interface.
For a complete description of the dial backup commands in this chapter, refer to the "Dial Backup Commands" chapter of the Dial Solutions Command Reference.
To configure a dial backup to a serial interface, you must configure the interface to use the dial backup interface, specify the conditions in which the backup interface will be activated, and then configure the dial-backup interface for DDR. The DDR configuration specifies the conditions and destinations for dial calls. The serial interface (often called the primary interface) might be configured for DDR or for Frame Relay or X.25 over a leased line, but the backup tasks are the same in all three cases.
To configure a backup interface for a serial interface based on one of the conditions above, complete the following general steps:
For a backup serial interface, an external data communications equipment (DCE) device, such as a modem attached to a circuit-switched service, must be connected to the backup serial interface. The external device must be capable of responding to a DTR Active signal by automatically dialing the preconfigured telephone number of the remote site.
You must decide whether to activate the backup interface when the primary line goes down, when the traffic load on the primary line exceeds the defined threshold, or both. The tasks you perform depend on your decision. Perform the tasks in the following sections as needed to configure dial backup:
Then configure the backup interface for DDR, so that calls are placed as needed. See the "Dial-on-Demand Routing" part of this manual for more information.
For simple configuration examples, see the "Dial Backup for Serial Interfaces Configuration Examples" section later in this chapter.
To specify a backup interface for a primary serial interface or subinterface, use one the following commands in interface configuration mode:
| Command | Purpose |
|---|---|
backup interface type number | Select a backup interface. |
When configured for Legacy DDR, the backup interface can back up only one interface. For examples of selecting a backup line, see the "Dial Backup Using an Asynchronous Interface Example" and the "Dial Backup Using DDR and ISDN Example" sections later in this chapter.
You can configure dial backup to activate the secondary line based on the traffic load on the primary line. The software monitors the traffic load and computes a 5-minute moving average. If this average exceeds the value you set for the line, the secondary line is activated and, depending upon how the line is configured, some or all of the traffic will flow onto the secondary dialup line.
To define how much traffic should be handled at one time on an interface, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
backup load {enable-threshold | never} | Define the traffic load threshold as a percentage of the primary line's available bandwidth. |
You can configure a value that defines how much time should elapse before a secondary line status changes after a primary line status has changed. This means that you can define two delays:
To define these delays, use the following command in interface configuration mode:
| Command | Purpose |
|---|---|
backup delay {enable-delay | never} | Define backup line delays. |
For examples of how to define backup line delays, see the sections "Dial Backup Using an Asynchronous Interface Example" and "Dial Backup Using DDR and ISDN Example" later in this chapter.
The following sections present examples of backup interfaces configured to be activated in three different circumstances:
The following is an example for dial backup using interface async 1, which is configured for DDR:
interface serial 0 ip address 172.30.3.4 255.255.255.0 backup interface async1 backup delay 10 10 ! interface async 1 ip address 172.30.3.5 255.255.255.0 dialer in-band dialer string 5551212 dialer-group 1 async dynamic routing ! dialer-list 1 protocol ip permit ! chat-script sillyman "" "atdt 5551212" TIMEOUT 60 "CONNECT" ! line 1 modem chat-script sillyman modem inout speed 9600
The following example of uses an ISDN interface to back up a serial interface.
Interface BRI 0 is configured to make outgoing calls to one number. This is a legacy DDR spoke example.
interface serial 1 backup delay 0 0 backup interface bri 0 ip address 1.2.3.4 255.255.255.0 ! interface bri 0 ip address 1.2.3.5 255.255.255.0 dialer string 5551212 dialer-group 1 ! dialer-list 1 protocol ip permit
The following example configures serial 1 as a backup line that becomes active only when the primary line (serial 0) goes down. The backup line will not be activated because of load on the primary line.
interface serial 0 backup interface serial 1 backup delay 30 60
The backup line is configured to activate 30 seconds after the primary line goes down and to remain on for 60 seconds after the primary line is reactivated.
The same example on the Cisco 7500 would be as follows:
interface serial 1/1 backup interface serial 2/0 backup delay 30 60
The following example configures the secondary line (serial 1) to be activated only when the load of the primary line reaches a certain threshold:
interface serial 0 backup interface serial 1 backup load 75 5
In this case, the secondary line will not be activated when the primary goes down. The secondary line will be activated when the load on the primary line is greater than 75 percent of the primary's bandwidth. The secondary line will then be brought down when the aggregate load between the primary and secondary lines fits within 5 percent of the primary bandwidth.
The same example on the Cisco 7500 would be as follows:
interface serial 1/1 backup interface serial 2/2 backup load 75 5
The following example configures the secondary line to activate once the traffic threshold on the primary line exceeds 25 percent:
interface serial 0 backup interface serial 1 backup load 25 5 backup delay 10 60
Once the aggregate load of the primary and the secondary lines return to within 5 percent of the primary bandwidth, the secondary line is deactivated. The secondary line waits 10 seconds after the primary goes down before activating, and remains active for 60 seconds after the primary returns and becomes active again.
The same example on the Cisco 7500 is as follows:
interface serial 1/0 backup interface serial 2/0 backup load 25 5 backup delay 10 60
Posted: Mon May 3 11:48:09 PDT 1999
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