What is a Demand Circuit?

OSPF can be tuned to better handle demand circuits. But what are demand circuits? Are they still relevant today?

This is a type a circuit that’s not always up. One that dials on demand. Think of old ISDN, x.25, and dial-up lines for example.

In the old days, these may have been expensive, and were only meant to be up when needed. Also, the amount of traffic that was sent over them also needed to be kept to a minumum.

OSPF caused problems with demand circuits. The regular hello messages cause the link to stay up. Additionally, the regular LSA refresh (every 30 minutes) caused the circuit to be used more heavily than desired.

 

 

OSPF With Demand Circuits

Back in 1995 (way back when I started high school!) RFC1793 was released. This included support for OSPF to be aware of demand circuits.

This feature is enabled per-interface (so it can be enabled selectively per neighbour). When two neighbours start an adjacency, and will negotiate the DC bit. If they both have this bit, then this feature is enabled. If successful, they then set the DNA (DoNotAge) bit in the LSA field.

 

This provides two major changes. Firstly, beyond the initial formation of the neighbour adjacency, hellos are suppressed. They’re not sent at all. This prevents OSPF from keeping a circuit up unnecessarily.

This does have a side effect. One neighbour won’t know (from an OSPF standpoint) when its neighbour goes down. The adjacency will remain up, even if the underlying circuit is down.

Note: Hello suppression applies to point-to-point and point-to-multipoint network types only. Hellos are not suppressed on other network types

 

The second change is that the periodic LSA refresh is also suppressed. This is because of the DNA bit I mentioned earlier. This means that the LSDB entries for this neighbour will not age out.

There will still be LSA refreshes, but now they will occur on demand, when there are changes in the network.

 

 

Is it Still Relevant Today?

We generally don’t use circuits like dial-up, and x.25, and ISDN is nearly extinct too. So, is this important at all?

Well, there are still some technologies that are on demand. Think of site-to-site VPNs for example. In particular, think of technologies like Cisco’s DMVPN (phase 2 or 3), or Juniper’s Auto Discovery VPN. These dynamically build tunnels between spoke routers on demand.

In cases like this, you may decide to this feature in OSPF. That is, of course, if the network type is point-to-point or point-to-multipoint (in an NBMA or broadcast network, OSPF will still send hellos).

 

 

Configuration

Cisco

interface gi0/0
  ip ospf demand-circuit

 

Juniper

set protocols ospf area 0 interface ge-0/0/0 demand-circuit

 

protocols {
  ospf {
    area 0.0.0.0 {
      interface ge-0/0/0 {
        demand-circuit;
      }
    }
  }
}

 

 

References

Cisco – OSPF Demand Circuit Feature

Juniper – Configuring OSPF Interfaces (Example: Configuring OSPF Demand Circuits)

IETF – RFC 1793: Extending OSPF to Support Demand Circuits