Connected: An Internet Encyclopedia
1.2. Definitions of commonly used terms

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1.2. Definitions of commonly used terms

1.2. Definitions of commonly used terms

This section provides definitions for terms that have a specific meaning to the OSPF protocol and that are used throughout the text. The reader unfamiliar with the Internet Protocol Suite is referred to [RS-85-153] for an introduction to IP.


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

Autonomous System

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

Interior Gateway Protocol

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.

Router ID

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


In this memo, 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.

Network mask

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

Multi-access networks

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).


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.

Neighboring routers

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.

Link state advertisement

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 forms the protocol's topological database.

Hello Protocol

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.

Designated Router

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.

Lower-level protocols

The underlying network access protocols that provide services to the Internet Protocol and in turn the OSPF protocol. Examples of these are the X.25 packet and frame levels for X.25 PDNs, and the ethernet data link layer for ethernets.

Next: 1.3. Brief history of link-state routing technology

Connected: An Internet Encyclopedia
1.2. Definitions of commonly used terms