IP Routing

Demand Circuits

The cost of a demand circuit, such as an ISDN link or a dial-up line, is dependent on its usage. It is desirable to use a demand circuit only for user traffic and not for overhead such as OSPF hellos or periodic LSAs. RFC 1793 describes modifications to OSPF that allow the support of demand circuits. This is an optional capability in OSPF; a router will set the DC bit in the options field if it supports the capability. Routers that support the capability will also set the high bit of the LS age field to 1 to indicate that the LSA should not be aged. This bit is also referred to as the do-not-age bit. OSPF demand circuits suppress periodic hellos and LSAs, but a topology change will still activate the demand circuit since LSA updates are required to keep the LS database accurate. Since any large network is likely to experience frequent topology changes, it may be prudent to define demand circuits in stub areas. Stub areas have a limited topology database and hence are shielded from frequent topology changes.

If a demand circuit is created in a stub area, all routers in the stub area must support the DC option -- routers that do not support demand circuits will misinterpret the age field (as the high bit is set). An LSA with the DC bit set to 1 is flooded into an area only if all LSAs in the database have their DC bits set to 1.

To configure an interface as a demand circuit, enter the following command in interface configuration mode on one end of the demand circuit: ...