OSPF Fundamental Terminology Explained

OSPF is a complex routing protocol. It uses many terms to describe its functions and operations. Learning these terms helps you manage it more effectively. This tutorial provides a detailed explanation of these terms.

Link

A link is a router's interface connected to an IP subnet. When you add an interface to the OSPF process, OSPF considers the interface as a link.

State

A link has two states: up and down. The up state shows the link (interface) is operational, and OSPF can reach the IP subnet connected to the link. The down state indicates that the link is not functional, and OSPF cannot reach the IP subnet connected to it.

Link-state protocol

OSPF is a link-state protocol. Link-state protocols use the Shortest Path First (SPF) algorithm to calculate the optimal path to a destination. To run this algorithm, link-state protocols learn the complete network topology. In an extensive network, this feature creates scalability problems. To solve this problem, OSPF uses two concepts: autonomous systems and areas.

Autonomous System

An autonomous system is a group of networks under a single administrative control, which can be a group of companies, a company, or a division within the company. There are two types of routing protocols: Interior Gateway Protocol (IGP) and Border Gateway Protocol (BGP). IGP routing protocols provide routing within a single AS. BGP routing protocols provide routing between different AS. OSPF is an IGP routing protocol. OSPF provides routing within a single AS.

Hierarchical Design and Areas

Within the AS, OSPF uses areas and hierarchical design. OSPF implements a two-layer hierarchy: backbone area and areas off the backbone. OSPF utilizes the backbone area to facilitate routing between areas off the backbone, controlling when and how much routing information is exchanged between routers.

An area is a group of contiguous networks. Each area uses a unique area ID. All routers in the same area use the same area ID. The following image shows how OSPF uses areas for hierarchical routing.

OSPF area concept

The following table lists essential terms used in the hierarchical design and the area concept.

Router ID (RID)

RID is the ID that OSPF uses to identify a router. OSPF uses the highest configured IP address as RID. If the router has loopback interface configurations, OSPF uses them to select RID. If not, OSPF uses all active physical interfaces to choose RID.

Neighbors

OSPF neighbors are two or more routers that have an interface in the same network and have the common configuration values. These configuration values are known as neighborship requirements.

Adjacency

An OSPF router shares routing updates only with its adjacent neighbors. An adjacency is a relationship between two adjacent routers that permits them to exchange routing updates directly.

Designated router (DR)

OSPF uses the concept of DR and BDR in a broadcast network to minimize the number of adjacencies formed. In a broadcast network, it designates one router as the DR. The designated router shares routing updates with all other routers.

Backup Designated Router (BDR)

A BDR is a hot standby router for the DR. The BDR maintains a backup copy of all databases running on DR. If DR fails, BDR immediately takes over as the primary DR.

Hello protocol

OSPF utilizes the Hello protocol to discover OSPF routers within the network and maintain relationships with its neighbors. Hello packets are sent to the multicast address 224.0.0.5.

OSPF database

OSPF maintains two types of databases: the neighborship database and the topological database. It uses the neighborship database to store a list of all OSPF routers. It uses the topological database to store information about all LSA packets.

Link State Advertisement (LSA)

LSA is an OSPF data packet containing link-state and routing information. OSPF routers share LSA packets only with adjacent routers.

LSDB

An LSDB is a collection of all LSAs received by the OSPF router. Each LSA has a unique sequence number. OSPF stores an LSA in LADB with its sequence number. Adjacent routers maintain the same LSDB.

Conclusion

Understanding the terminology and concepts related to OSPF is crucial for effective network management and optimization. By grasping key terms such as links, states, routers, areas, and various routing processes, you can configure, troubleshoot, and maintain your OSPF implementations.