OSPF Network Types
Figure 4-3 illustrates the three different network types within which OSPF operates.
The following list explains the physical characteristics of the OSPF network types illustrated in Figure 4-1:
Every router running OSPF within a network must have a unique router ID. This identification number is a 32-bit number that identifies one router to another router within an Autonomous System (AS). The router ID is used by the OSPF link-state database (LSDB) as a method of tracking each router within the AS and the links associated with it.
This identification number is unique to each OSPF router. You can employ a couple different methods to determine how your network decides upon the OSPF router ID.
To assign the router ID, OSPF uses the default method of determining the highest IP address on one of the routers active interfaces.
The other method involves manually assigning the router ID number by configuring a loopback address on the Cisco router in question. This method has the added benefit of being much more stable than the default method because a loopback address cannot go down or lose connectivity, which would result in the need to update routing tables. Chapter 7, Designing & Implementing an OSPF Network, discusses loopback addresses in greater detail.
Configuring a loopback address as the OSPF router ID has a very significant benefit in its stability. The interface is essentially a software-based interface that can be used for many additional purposes such as summarizing IP address ranges or troubleshooting. They are reachable, provided they fall within the advertised IP address category.
OSPF considers two routers that have an interface located on a common network as neighbors. When OSPF discovers its neighbors, this is the first step of discovering the network and building a routing table. This process begins by the router learning the router identification numbers. On multiaccess networks, these neighbors are dynamically discovered by the OSPF Hello protocol, which will be discussed in later chapters.
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 will form an adjacency. The six conditions under which OSPF will form adjacencies are as follows:
Adjacencies control the distribution of routing updates in the sense that only routers adjacent to the one sending an update will process it.
Designated Routers (DRs)
OSPF builds adjacencies between routers for purposes of exchanging routing information. When OSPF has to deal with non-broadcast multiaccess (NBMA) or broadcast networks, however, a problem presents itself. In these types of networks, there are multiple routers, which would result in entirely too many adjacencies. To combat superfluous adjacencies, the Designated Router is introduced.