As per RFC 1793,extending OSPF to support demand cicuits,"OSPF hellos and the refresh of OSPF routing information are suppressed on demand circuits,allowing the underlying data-link connection to be closed when not carrying application traffic".This feature allows low speed and pay per use link such as analod dial,ISDN etc. to run OSPF without the need for periodic hellos and LSA flooding.
This feature is enabled with the interface level command "ip ospf demand-circuit".A demand circuit that goes up and down indicates a problem that needs to be investigated.
The most common reasons for this problem consist of the following:
- A change is observed in the network topology solution.
- The network type is defined as a broadcast solution.
- One or more routers do not understand the demand circuit.
- The host route is redistributed into the Open Shortest Path First (OSPF) database.
- The OSPF demand circuit is configured over an asynchronous interface.
- The OSPF demand circuit is configured over a Multilink PPP (MPPP).
For information on why OSPF keeps bringing up the link, refer to Why OSPF Demand Circuit Keeps Bringing Up the Link.