Dial-on-demand routing (DDR) is common in scenarios in which the ISDN or similar dialup links are used as a backup link. When the primary link goes down, this backup link comes up. RIP begins sending and receiving updates on this link as long as the primary link is down.
The dialup links can be used as a backup for the primary link in two ways:
• Use the backup interface command.
• Use a floating static route with a dialer list that defines interesting traffic.
The first method is very simple: The command is typed under the dial interface, indicating that it's a backup for a primary interface.
The second method requires a floating static route with a higher administrative distance than RIP (for example, 130 or above). It also requires defining interesting traffic that should bring up the link. The RIP broadcast address of 255.255.255.255 must be denied in the dialer list, so it shouldn't bring up the link unnecessarily.
When running RIP under DDR situations, there are a number of issues to consider. Some problems are related to the ISDN line or an async line in which RIP updates keep bouncing. Some problems are related to the configuration. This section talks about the two most common dialup problems:
• A RIP broadcast is keeping the link up.
• RIP updates are not going across the dialer interface.
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