Core Issue
When configured for Open Shortest Path First (OSPF), an interface can belong to one area only. When legacy ISDN is used for backup in an OSPF network, the area border router must have one PRI or BRI interface for every area for which backup is configured. This results in inefficient utilization of the B channels. Dialer profiles, which separate the logical configuration from the physical interface, can better utilize the B channels on BRI and PRI interfaces. However, the use of dialer profiles for OSPF backup suffers from these shortcomings:
- The generation of extra Link-State Advertisements (LSAs).
- Areas become discontiguous.
- The introduction of additional routes.
- Frequent triggering of ISDN calls for each change in an LSA database.
- The triggering of ISDN calls due to periodic LSA flooding.
Resolution
To avoid inefficient utilization of the available B channels, use the ISDN virtual profile feature. Using this feature, the B channels belonging to a single interface can be used to configure backup for multiple areas.
For more information on configuring ISDN backup for OSPF networks using virtual profiles, refer to Scalable ISDN Backup Strategy for Large OSPF Networks.
For more information on virtual profiles, refer the How Virtual Profiles Work -- Four Configuration Cases section of Configuring Virtual Profiles.
