OSPF Demand Circuit Explained
When deploying OSPF (Open Shortest Path First) on on-demand links like ISDN, unnecessary hello traffic can keep the circuit active, increasing cost or overhead. The demand circuit feature suppresses OSPF hello packets, ensuring the link only activates when actual routing updates or data traffic must traverse it. For background on OSPF, see OSPF on Wikipedia.
The Dial Interface Challenge
Dial technologies (ISDN, etc.) are often billed per minute. Default OSPF hellos and LSA refreshes keep the line unnecessarily open. Demand circuits avoid this, reducing both cost and overhead.
How the Configuration Works
- Configure PPP encapsulation and authentication for secure link negotiation.
- Use dialer maps and dialer groups to define call behavior.
- Set ISDN switch-type and SPIDs as required by the carrier.
- Enable
ip ospf demand-circuitto suppress unnecessary hellos.
Only one side of the link needs the demand-circuit command for it to function.
The Evolution of the Feature
- Smarter hello suppression keeps the OSPF adjacency virtually up without constant hellos.
- DoNotAge (DNA) LSAs reduce unnecessary LSA refreshes.
- Dialer integration allows precise control of traffic that triggers the link.
Interactive Diagram: Demand Circuit Behavior
graph TD
RouterA[Dial Router A]
RouterB[Dial Router B]
Circuit[ISDN Demand Circuit]
RouterA --> Circuit
RouterB --> Circuit
Circuit --> OSPF[OSPF Area]
%% Styling
classDef demand fill:#fdd,stroke:#d00,stroke-width:2px;
classDef active fill:#dfd,stroke:#080,stroke-width:2px;
class Circuit demand;
class RouterA,RouterB active;
Red node represents the demand circuit (suppressed hello traffic). Green nodes are routers actively participating in OSPF adjacencies. The circuit only activates when data or updates need to traverse the link.
Practical Takeaways
- Only one side requires
ip ospf demand-circuit. - The adjacency remains virtually up even if the physical link is idle.
- Useful for backup, low-use, or metered links.
- Dialer lists control which traffic triggers the link.
Conclusion
OSPF demand circuits allow dynamic routing over on-demand links without unnecessary protocol chatter. The link remains efficient and only comes up when required, preserving OSPF stability while minimizing cost and overhead. This approach remains relevant for backup, satellite, or any metered connection.
