What do you do if you have more traffic than bandwidth? Wouldn’t it be great if you could pull
your magic router wand out and make the traffic go faster? You can approximate this magic by
using Bandwidth on Demand.
Bandwidth on Demand (BoD) is an interface-only command, meaning you cannot apply it
to a subinterface. Here is the syntax to assign a backup load to an interface:
backup load {enable-threshold | never} {disable-load | never}
The enable threshold load is the percentage of interface load where you want the additional
bandwidth dialed up. The disable load is the percentage of interface load where you want the
extra bandwidth dropped. At what point is the circuit congested enough to need extra bandwidth?
Some people say 75 percent; yet others say queuing is needed. You will probably have
to figure this out based on corporate policy, cost, sensitivity to slow responsiveness, and so on.
Because BoD is a dial-up feature, you might incur additional long-distance costs, so be careful
about setting your thresholds.
Configuring BoD is almost the same as configuring dial backup, except you’re replacing the
amount of backup delay with the amount of backup threshold. Here is an example:
Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#interface serial0
Router(config-if)#backup interface BRI0
This configuration sets the interface serial0 to use interface BRI0 as a backup as the main
interface goes down. The following configuration shows how to configure the backup delay and
the backup load:
Router(config-if)#backup ?
delay Delays before backup line up or down transitions
interface Configure an interface as a backup
load Load thresholds for line up or down transitions
Router(config-if)#backup delay ?
<0-4294967294> Seconds
never Never activate the backup line
Router(config-if)#backup delay 10 ?
<0-4294967294> Seconds
never Never deactivate the backup line
Router(config-if)#backup delay 10 60
The previous configuration sets the backup delay to 10 seconds and 60 seconds. This means
that the backup interface will not dial until serial0 is down for 10 seconds, and it will drop the link
after the serial link is back up for 60 seconds. The backup load command syntax is as follows:
Router(config-if)#backup load ?
<0-100> Percentage
never Never activate the backup line
Router(config-if)#backup load 75 ?
<0-100> Percentage
never Never deactivate the backup line
Router(config-if)#backup load 75 35
Router(config-if)#^Z
Router#
This command sets the router to dial the ISDN BRI0 interface if the bandwidth reaches a
maximum of 75 percent and then to drop the link after the bandwidth is back at 35 percent.
The interface configuration is shown next:
Router#show run
[output cut]
interface Serial0
backup delay 10 60
backup interface BRI0
backup load 75 35
ip address 10.53.69.69 255.255.255.0
no ip directed-broadcast
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