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OSPF

  • Article

Applies To: Windows Server 2003, Windows Server 2003 R2, Windows Server 2003 with SP1, Windows Server 2003 with SP2

OSPF

Open Shortest Path First (OSPF) is designed for exchanging routing information within a large or very large internetwork. This feature is not available on the Itanium-based versions of the Windows operating systems. This content is not available in this preliminary release.

The biggest advantage of OSPF is that it is efficient; OSPF requires very little network overhead even in very large internetworks. The biggest disadvantage of OSPF is its complexity; OSPF requires proper planning and is more difficult to configure and administer.

OSPF uses a Shortest Path First (SPF) algorithm to compute routes in the routing table. The SPF algorithm computes the shortest (least cost) path between the router and all the networks of the internetwork. SPF-calculated routes are always loop-free.

Instead of exchanging routing table entries like RIP routers, OSPF routers maintain a map of the internetwork that is updated after any change to the network topology. This map, called the link state database, is synchronized between all the OSPF routers and is used to compute the routes in the routing table. Neighboring OSPF routers form an adjacency, which is a logical relationship between routers to synchronize the link state database.

Changes to internetwork topology are efficiently flooded across the entire internetwork to ensure that the link state database on each router is synchronized and accurate at all times. Upon receiving changes to the link state database, the routing table is recalculated.

As the size of the link state database increases, memory requirements and route computation times increase. To address this scaling problem, OSPF divides the internetwork into areas (collections of contiguous networks) that are connected to each other through a backbone area. Each router only keeps a link state database for those areas that are connected to the router. Area border routers (ABRs) connect the backbone area to other areas.

To further reduce the amount of routing information flooded into areas, OSPF allows the use of stub areas. A stub area can contain a single entry and exit point (a single ABR), or multiple ABRs when any of the ABRS can be used to reach external route destinations.

The following illustration shows a diagram of an OSPF internetwork.

OSPF internetwork with two areas and backbone area

OSPF has the following advantages over RIP:

  • OSPF-calculated routes are always loop-free.

  • OSPF can scale to large or very large internetworks.

  • Reconfiguration for network topology changes is faster.

The implementation of OSPF in Routing and Remote Access has the following features:

  • Route filters for controlling interaction with other routing protocols.

  • Dynamic reconfiguration of all OSPF settings.

  • Coexistence with RIP.

  • Dynamic addition and deletion of interfaces.

For information about designing and deploying an OSPF internetwork, see Setting Up an OSPF Routed Internetwork.

For detailed information about the operation of OSPF, see "Unicast IP Routing" at the Microsoft Windows Resource Kits Web site.

Notes

  • Routing and Remote Access does not support the use of OSPF in a demand-dial configuration that uses nonpermanent, dial-up links.

  • If you are using multiple IP routing protocols, configure only a single routing protocol per interface.