A level three Internet Protocol packet switch. Formerly called a gateway in much of the IP literature.

A group of routers exchanging routing information via a common routing protocol. Abbreviated as AS.

The routing protocol spoken by the routers belonging to an Autonomous system. Abbreviated as IGP. Each Autonomous System has a single IGP. Separate Autonomous Systems may be running different IGPs.

A 32-bit number assigned to each router running the OSPF protocol. This number uniquely identifies the router within an Autonomous System.

In this documentation, an IP network/subnet/supernet. It is possible for one physical network to be assigned multiple IP network/subnet numbers. We consider these to be separate networks. Point-to-point physical networks are an exception - they are considered a single network no matter how many (if any at all) IP network/subnet numbers are assigned to them.

A 32-bit number indicating the range of IP addresses residing on a single IP network/subnet/supernet. This specification displays network masks as hexadecimal numbers. For example, the network mask for a class C IP network is displayed as 0xFFFFFF00. Such a mask is often displayed elsewhere in the literature as 255.255.255.0.

Those physical networks that support the attachment of multiple (more than two) routers. Each pair of routers on such a network is assumed to be able to communicate directly (e.g., multi-drop networks are excluded).

OSPF allows collections of contiguous networks and hosts to be grouped together. Such a group, together with the routers having interfaces to any one of the included networks, is called an area. Each area runs a separate copy of the basic link-state routing algorithm. This means that each area has its own topological database and corresponding graph. The topology of an area is invisible from the outside of the area. Conversely, routers internal to a given area know nothing of the detailed topology external to the area. This isolation of knowledge enables the protocol to effect a marked reduction in routing traffic as compared to treating the entire Autonomous System as a single link-state domain.

This is a 32-bit number that identifies the area. The Area ID of 0.0.0.0 is reserved for the backbone. If the area represents a subnetted network, the IP network number of the subnetted network may be used for the Area ID.

The backbone area consists of those networks not contained in any area, their attached routers, and those routers that belong to multiple areas. The backbone must be contiguous.

It is possible to define areas in such a way that the backbone is no longer contiguous. In this case, the system administrator must restore backbone connectivity by configuring virtual links. Virtual links can be configured between any two backbone routers that have an interface to a common non-backbone area. Virtual links belong to the backbone. The protocol treats two routers joined by a virtual link as if they were connected by an unnumbered point-to-point network. On the graph of the backbone, two such routers are joined by arcs whose costs are the intra-area distances between the two routers. The routing protocol traffic that flows along the virtual link uses intra-area routing only.

The connection between a router and one of its attached networks. An interface has state information associated with it, which is obtained from the underlying lower level protocols and the routing protocol itself. An interface to a network has associated with it a single IP address and mask (unless the network is an unnumbered point-to-point network). An interface is sometimes also referred to as a link.

Two routers that have interfaces to a common network. On multi-access networks, neighbors are dynamically discovered by OSPF's Hello Protocol.

A relationship formed between selected neighboring routers for the purpose of exchanging routing information. Not every pair of neighboring routers become adjacent.

Describes the local state of a router or network. This includes the state of the router's interfaces and adjacencies. Each link state advertisement is flooded throughout the routing domain. The collected link state advertisements of all routers and networks from the protocol's topological database.

The part of the OSPF protocol used to establish and maintain neighbor relationships. On multi-access networks, the Hello Protocol can also dynamically discover neighboring routers.

Each multi-access network that has at least two attached routers has a Designated Router. The Designated Router generates a link state advertisement for the multi-access network and has other special responsibilities in the running of the protocol. The Designated Router is elected by the Hello Protocol. The Designated Router concept enables a reduction in the number of adjacencies required on a multi-access network. This in turn reduces the amount of routing protocol traffic and the size of the topological database.

In order to make the transition to a new Designated Router smoother, there is a Backup Designated Router for each multi-access network. The Backup Designated Router is also adjacent to all routers on the network and becomes Designated Router when the previous Designated Router fails. If there were no Backup Designated Router, when a new Designated Router became necessary, new adjacencies would have to be formed between the new Designated Router and all other routers attached to the network.

A network that joins a single pair of routers. A 56Kb serial line is an example of a point-to-point network.

Networks supporting many (more than two) attached routers, together with the capability to address a single physical message to all of the attached routers (broadcast). Neighboring routers are discovered dynamically on these nets using OSPF's Hello Protocol. The Hello Protocol itself takes advantage of the broadcast capability. The protocol makes further use of multicast capabilities if they exist. An Ethernet is an example of a broadcast network.

Networks supporting many (more than two) routers, but having no broadcast capability. Neighboring routers are also discovered on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configuration information is necessary for the correct operation of the Hello Protocol. On these networks, OSPF protocol packets that are normally multicast need to be sent to each neighboring router, in turn. An X.25 Public Data Network (PDN) is an example of a non-broadcast network.

A router with all directly connected networks belonging to the same area. Routers with only backbone interfaces also belong to this category. These routers run a single copy of the basic routing algorithm.

A router that is attached to multiple areas. Area border routers run multiple copies of the basic algorithm, one copy for each attached area and an additional copy for the backbone. Area border routers condense the topological information of their attached areas for distribution to the backbone. The backbone in turn distributes the information to the other areas.

A router that has an interface to the backbone. This includes all routers that interface to more than one area (i.e., area border routers). However, backbone routers do not have to be area border routers. Routers with all interfaces connected to the backbone are considered to be internal routers.

A router that exchanges routing information with routers belonging to other Autonomous Systems. Such a router has AS external routes that are advertised throughout the Autonomous System. The path to each AS boundary router is known by every router in the AS. This classification is completely independent of the previous classifications: AS boundary routers may be internal or area border routers, and may or may not participate in the backbone.

Hosts attached directly to routers (referred to as host routes) appear on the graph as stub networks. The network mask for a host route is always 0xFFFFFFFF, which indicates the presence of a single node.

In some Autonomous Systems, the majority of the topological database may consist of AS external advertisements. An OSPF AS external advertisement is usually flooded throughout the entire AS. However, OSPF allows certain areas to be configured as "stub areas". AS external advertisements are not flooded into/throughout stub areas; routing to AS external destinations in these areas is based on a (per-area) default only. This reduces the topological database size, and therefore the memory requirements, for a stub area's internal routers. The OSPF protocol ensures that all routers belonging to an area agree on whether the area has been configured as a stub. This guarantees that no confusion will arise in the flooding of AS external advertisements. There are a couple of restrictions on the use of stub areas. Virtual links cannot be configured through stub areas. In addition, AS boundary routers cannot be placed internal to stub areas.

An OSPF area is defined as a list of address ranges. Each address range consists of the following items: