In today’s internetwork environment, most networks are required to be up and operating efficiently 24 hours a day, seven days a week. However, let’s face it; no network has 100 percent uptime. To ensure 24×7 connectivity, backup solutions must be in place. Unfortunately, for most companies, purchasing a redundant, dedicated WAN connection is simply cost prohibitive. Besides, why pay for a dedicated link that, with luck, will rarely be used? That’s where Cisco’s Dial-on-Demand Routing (DDR) comes in.

Primary link failure
There are two methods for establishing a DDR backup link. The first is to use a static route. The static route feature on Cisco routers allows you to use a DDR connection to back up a primary connection. To implement a DDR backup route, simply define a static route to a DDR interface (usually an ISDN interface) and set an administrative distance for the DDR route to a number higher than that of the primary route. When the primary link fails, the static route to the DDR interface will be the successor, and a DDR link will be established. Because the primary route has a lower administrative cost, it will again be chosen as the preferred route when it comes back online, and the DDR link will be terminated.

The second method is Dial Backup. Using the “backup interface” command from the interface configuration mode prompt, you can back up to a primary link in case of failure. The Dial Backup feature also gives you the added benefit of selecting the amount of time a link must be down before a DDR link is established and the amount of time the primary link must be back up before the DDR link is terminated. These features can be especially helpful if the primary network link is going up and down sporadically.

Primary link saturation
Many times, WAN connectivity is interrupted not because of the failure of a link, but simply because the link has been saturated with traffic. If link saturation is a continual problem, the only solution is to increase the size of your WAN connection. However, if WAN saturation is sporadic—let’s say you have a problem only when your NT administrator is downloading the latest service pack from Microsoft—then DDR can help.

Using the “backup interface” command from the interface configuration mode prompt, you can configure back up to a saturated link. Via this command, you can set the level of saturation that will cause a DDR link to be established or terminated. For example, a DDR link can be set to connect when the primary link is 80 percent saturated and to disconnect when the primary link falls below a 50 percent saturation level.

DDR can be your high-speed backup solution
Using ISDN PRI, a DDR link can support up to T-1 speeds at a fraction of the cost of a dedicated link. For more information on DDR and other remote access solutions, check out Building Cisco Remote Access Networks.

Warren Heaton CCDA, CCNA, MCSE+I is the Cisco Program Manager for A Technological Advantage in Louisville, KY.

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