How to configure ISDN backup in large OSPF networks

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:

	Areas become discontiguous.
	The introduction of additional routes.
	The triggering of ISDN calls due to periodic LSA flooding.
	The generation of extra Link-State Advertisements (LSAs).
	Frequent triggering of ISDN calls for each change in an LSA database.

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 regarding configuring ISDN backup for OSPF networks using virtual profiles refer to:

Scalable ISDN Backup Strategy for Large OSPF Networks

For more information regarding virtual profiles, refer to:

How Virtual Profiles Work—Four Configuration Cases

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