18 Open Shortest Path First

This chapter covers

  • Open Shortest Path First link-state advertisements and database
  • How OSPF routers calculate routes
  • Configuring OSPF on Cisco routers
  • How OSPF routers become neighbors and form adjacencies

Open Shortest Path First (OSPF) is an interior gateway protocol (IGP) that serves as a key building block of modern enterprise networks. In chapter 17, we covered dynamic routing protocols in general and also examined how to use the network command to activate OSPF on router interfaces. In this chapter, we will dig deeper into the topic of OSPF and see how it actually works, including how OSPF-enabled routers become neighbors with each other, share routing information, calculate routes, and many other details.

These days, two versions of OSPF are in use: OSPFv2, which is primarily used for IPv4 networks, and OSPFv3, which is primarily used for IPv6 networks (although it can be used for IPv4 as well). For the purpose of the CCNA exam, the version we are concerned with is OSPFv2; all mentions of OSPF in this book are specifically referring to OSPFv2, as stated in exam topic 3.4: Configure and verify single area OSPFv2.

18.1 OSPF foundations

18.1.1 The link-state database

18.1.2 OSPF areas

18.1.3 OSPF cost

18.2 OSPF configuration

18.2.1 The router ID

18.2.2 Activating OSPF on interfaces

18.2.3 Passive interfaces

18.2.4 Advertising a default route

18.3 Neighbors and adjacencies

18.3.1 Neighbor states

18.3.2 OSPF network types