OSPF (Open Shortest Path First) Protocol

CCNA exam objectives include the OSPF (Open Shortest Path First) protocol. This tutorial series explains all the concepts, commands, and configurations of OSPF you need for the CCNA exam in detail.

OSPF CCNA Exam Objectives

CCNA exam objectives include the following OSPF-related topics.

  • Configure and verify single area OSPFv2
  • Neighbor adjacencies
  • Point-to-point
  • Broadcast (DR/BDR selection)
  • Router ID

OSPF Tutorials

The following tutorial series explains the above CCNA objectives and all other OSPF-related topics in detail.

What is OSPF?

Routers connect different IP subnets in the network. To connect subnets, they learn all network paths, select the best path for every subnet from all available paths, and save the chosen path in the routing table.

A routing protocol automates this process. There are many routing protocols. Based on how they calculate the best path, they are categorized into three types: distance vector, link state, and hybrid.

The thing routing protocols use to calculate the best and shortest path is known as the metric. Distance vector routing protocols use the number of routers in the path as the metric. RIPv1 and RIPv2 are distance vector routing protocols. Hybrid routing protocol uses bandwidth and delay as the metric. EIGRP is a hybrid routing protocol. Link State routing protocols use path cost as the metric. OSPF is a Link State routing protocol.

OSPF Overview

OSPF stands for Open Shortest Path First. It is described in RFC 2328. It is an open standard routing protocol. It dynamically learns all network paths, selects the best path for every subnet, and adds that to the routing table. It uses path cost to compare and choose the shortest path from all available paths.

OSPF Standards

OSPF is based on open standards. It runs on all routers. If a network is using OSPF routing protocol, it can have routers from any vendor. For example, it can have some routers from Cisco while remaining from Juniper.

OSPF Versions

OSPF has three versions: OSPFv1, OSPFv2, and OSPFv3. OSPFv1 was developed in the mid-1980s to overcome limitations, deficiencies, and scalability problems of the routing protocols of that time. In 1998, OSPFv1 was updated by OSPFv2 to support modern infrastructure and networks.

OSPFv2 includes all the features and characteristics that modern networks need. It is one of the most popular and widely used routing protocols. It was developed when IPv6 was not in use. Because of this, support for IPv6 was not added to OSPFv2.

To support IPv6, instead of updating the OSPFv2, developers developed a new version of OSPF. It was OSPFv3. It was released in 2008. The following table compares the features of OSPFv2 and OSPFv3.

OSPFv2 OSPFv3
It supports IPv4. It supports IPv6.
It is specified in RFC2328. It is specified in RFC 5340.
The header size is 24 bytes. The header size is 16 bytes.
It uses seven link-state advertisements. It uses nine link-state advertisements. Two new included LSA are link-LSA and intra-area-prefix-LSA.
You can run only one instance per link. You can run many instances per link.
It needs a network mask to form an adjacency. It does not need a network mask to form an adjacency.
It uses MD5 hashing for authentication. It uses IPSec for authentication.
It uses networks. It uses links.
It can configure its RID automatically. It can't configure its own RID. You have to configure RID manually.

Since OSPFv1 has been updated and replaced by OSPFv2, network administrators commonly use the term OSPF to refer to OSPFv2. Because of this, unless the version of OSPF is explicitly mentioned, you can consider all references to OSPF to be OSPFv2.

ComputerNetworkingNotes CCNA Study Guide OSPF (Open Shortest Path First) Protocol