Supress unneccessary LSA updates and traffic

OSPF demand circuit options suppresses hello and LSA refresh functions. OSPF can establish a demand link to form an adjacency and perform initial database synchronization, the adjacency remains active even after Layer 2 of the demand circuit goes down.

How Is OSPF over Demand Circuit Different from a Normal Circuit?

There are two main features of OSPF over demand circuit that make it different from a normal circuit.

• Suppressed periodic hellos

• Suppressed periodic LSA refresh

Suppressed Periodic Hellos

When an OSPF demand circuit is configured on a link, the periodic OSPF hellos are suppressed. Periodic hellos are suppressed only on a point-to-point and point-to-multipoint network type. On any other network type, OSPF hellos are still sent over the interface.

Suppressed Periodic LSA Refresh

Periodic LSA refreshes that take place every 30 minutes do not occur with OSPF demand circuit. When a demand circuit link is established a unique option bit (the DC bit) is exchanged between neighboring routers. If two routers negotiate the DC bit successfully they make a note of it and set a specific bit in the LSA Age called the DoNotAge bit (DNA). The DNA bit is the most significant bit in the LS Age field. By setting this bit the LSA stops aging, and no periodic updates are sent.

When Is a Periodic LSA Refresh Sent over an OSPF Demand Circuit?

There are only two scenarios where the periodic LSA refresh occurs when using the OSPF demand circuit feature:

• If there is a change in network topology

• If there is a router in the OSPF domain that can not understand demand circuits