OSPF: Difference between revisions

* What is a Demand Circuit?

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Source: [https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/5132-dc.html cisco.com]

Demand circuit options were introduced for Open Shortest Path First (OSPF) in Cisco IOS® Software release 11.2 in response to the OSPF RFC 1793 leavingcisco.com.

OSPF sends hellos every 10 seconds and refreshes its link-state advertisements (LSAs) every 30 minutes.

These functions maintain neighbor relationships and ensure that the link-state databases are accurate and use far less bandwidth than similar functions in Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP).

However, even this amount of traffic is undesirable on demand circuits. Using 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.

Cisco IOS version 12.1(2)T introduces the flooding reduction feature for OSPF.

This feature is intended to minimize traffic created by a periodic refresh of LSAs in OSPF domains with large number of LSAs.

Unlike the OSPF demand circuit feature, flooding reduction is usually configured on leased lines.

Flooding reduction uses same technique as demand circuits to suppress the periodic LSA refresh.

This feature is submitted for standardization into the IETF OSPF working group.

The main difference between flooding reduction and demand circuits is that former suppresses only periodic LSA refreshes; it does not suppress periodic hello packets.

Thus, the flooding reduction feature does not impair the detection of a neighbor router going down.

ip ospf demand-circuit