For adjacencies to form, OSPF must first have discovered its neighbors. Adjacencies are formed for the purpose of exchanging routing information. Not every neighboring router forms an adjacency. The conditions under which OSPF forms adjacencies are as follows:
- Network connectivity is point-to-point.
- Network connectivity is achieved through a virtual link.
- The router is the DR.
- The neighboring router is the DR.
- The router is the BDR.
- The neighboring router is the BDR.
Adjacencies control the distribution of routing updates in the sense that only routers adjacent to the one sending an update process the update.
Neighbor Versus Adjacent OSPF Routers
This is a common question that should be explained easily yet is often not. Think of it this way. You live next to many different people, and you are considered neighbors to them as a result. You might wave at them when you drive to the store, but you are not close friends.
In certain circumstances, you become close friends with people with whom you were first neighbors. These close friends become known as adjacent; you talk to each other over the fence in the summer, have them over for dinner-that kind of thing. You essentially have a stronger level of communication with them.
In this tutorial:
- OSPF Basics
- What Is a Routing Protocol?
- Basic Routing Protocol Operation
- Link-State Versus Distance Vector Routing Protocols
- Link-State Routing Protocols
- OSPF Characteristics
- Integrated Intermediate System-to-Intermediate System
- Distance Vector Routing Protocols
- Selecting a Routing Protocol
- Operational Considerations
- Protocols Supported
- Routing Hierarchies
- IP Address Management
- IP Encapsulation Support
- Technical Considerations
- Routing Updates
- Business Considerations
- SPF Overview
- OSPF Routing Hierarchy
- Hierarchical Network Design Techniques
- Routing Types Within an OSPF Network
- External Routes
- OSPF Areas
- Characteristics of a Standard OSPF Area
- Stub Areas
- Not-So-Stubby Areas
- OSPF Operational Environment
- Types of OSPF Routers
- OSPF Network Types
- Router Identification