Troubleshooting Dial-on-Demand Routing (DDR) Issues in IGRP

Dial-on-demand routing is very common when the ISDN or similar dialup links are used as a backup link. When the primary link goes down, this backup link comes up. IGRP starts sending and receiving updates on this link as long as the primary link is down.

Two ways exist for using the dialup links as a backup for the primary link:

• Using the backup interface command

• Using floating static routes with a dialer list that defines interesting traffic

The first method is simple: The command is typed under the dial interface indicating that it is a backup for a primary interface.

The second method requires a floating static route with a higher administrative distance than IGRP ‚ for example, 110 or above. It also requires defining interesting traffic that should bring up the link. The IGRP broadcast address of 255.255.255.255 must be denied in the dialer list, so it should not bring up the link unnecessarily.

When running IGRP under dial backup situations, a lot of issues must be considered . Some problems are related to the ISDN line or async line that keeps coming up. Some problems are related to the configuration. This section talks about the two most common dial backup problems:

• IGRP broadcast is keeping the link up.

• IGRP updates are not going across dialer interface.