On-Demand Routing

On-demand routing (ODR) is a way to provide for on-demand circuits (such as dial-on-demand ISDN circuits) within an OSPF autonomous system. Because OSPF generally uses hello packets and periodic reflooding of LSAs to maintain network state, it would normally be impossible to run this protocol over a dial-on-demand circuit because the circuit would need to remain up at all times.

To resolve this, OSPF allows a special bit to be set within the advertisement, which indicates this LSA should never be aged out. This allows two routers connected over an on-demand circuit to exchange databases when the circuit is up and not lose information about destinations across the link when it is down.

ODR is relatively simple to configure—the one caveat is that all routers in the area must support ODR (even if they don't have it configured) so that they will understand the special bit settings in the LSA. Routers that aren't ODR capable will simply time the routes out as usual, and the network will periodically lose connectivity to any destinations beyond the dial-on-demand link.