A Comparison of Dial Backup Approaches
- The Tradeoffs
When implementing a dial backup solution for improving network availability, three basic approaches are available. In the Cisco world, they're called backup interface, dial-on-demand, and dialer watch. Each mechanism has advantages and disadvantages, making an understanding of their strengths and weaknesses critical when selecting which approach to use for a specific design.
Dial backup is frequently used as a low-cost method to improve network availability. However, care must be taken to ensure that the backup link will be activated whenever it's needed; otherwise, the availability improvement may be illusory.
The standard Cisco backup interface command set activates the backup link whenever the interface to the link being backed up is detected to be down. Backup interface is a proven methodology that works very well with point-to-point links and other applications in which the link protocol can reliably detect link failures. However, in the event of a failure that prevents communications but leaves the local line protocol up, such as a switch failure inside the frame relay network, the backup commands are not activated and communications remain down.
Dial-on-demand routing also has a long history of usage in routed networks. Dial-on-demand routing is designed to provide cost-effective connectivity in dial-up environments where the goal is to minimize the time spent with the line engaged. Filters are used to define "interesting traffic." Calls are connected when the dial link is determined to be the best route to the destination for the interesting traffic and dropped when no interesting traffic has used the dial link for a specified period.
The challenge is that backup interface only works when a link failure is detected at the link level. Meanwhile, dial-on-demand routing only works when interesting traffic is present. If traffic suitable for forcing the link up cannot be guaranteed, the link may not be up when needed. At the same time, care must be taken to ensure that the dial-up link will only be the best path for interesting traffic when the primary link is down. Otherwise the backup link will be kept up unnecessarily, wasting money and potentially preventing its use for backing up a link that does need support.
Cisco has recognized these shortcomings and introduced the dialer watch feature in IOS 12.0 to provide the ability to trigger backup action based on changes in the routing tables. Theoretically, dialer watch is the ideal solution to the dial backup challenge. However, it suffers from its own shortcomings:
It's only available on Cisco routers.
It's a relatively new capability, so support staff may be less familiar with it.
If the calling router is not already at IOS 12.0 or later, an upgrade is required.
With a little extra effort, dial-on-demand routing can be used to provide most of the dialer watch functionality using any vendor's routers or any supported IOS release on Cisco routers. The trick is to use floating static routes to trigger dial-on-demand routing to bring up the backup link. This makes the implementation a little more complex, but benefits from using only proven, well-debugged capabilities.