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CCNP Routing Studies: Basic Open Shortest Path First

Chapter Description

Understanding basic Open Shortest Path First (OSPF) routing principles not only applies to the CCNP certification but to all Cisco-based certifications. A concrete understanding of how OSPF routing works is fundamental for any small or large network. This chapter focuses on a number of objectives falling under the CCNP routing principles.

OSPF and Nonbroadcast Multiaccess Environments

A nonbroadcast multiaccess (NBMA) environment presents the OSPF designer a number of challenges. The main challenge is that NBMA environments do not carry broadcast traffic but have the added characteristics that multiple destinations may be present. In a normal broadcast environment, this is not a challenge because a packet can be sent to a broadcast or multicast address and be received by all recipients.

To overcome these problems, OSPF, and in particular Cisco IOS, allows you to define the networks types and also allows static OSPF neighbor configurations.

Cisco IOS enables you to configure five main network types as displayed in Table 3-2. These five possible solutions available with Cisco IOS are listed for your reference.

Table 3-2 OSPF over NBMA Using Cisco IOS



Point-to-point nonbroadcast

Used typically for Frame Relay interfaces.


This is the default mode for subinterfaces.


Used for multiple destinations.


NBMA mode.


Used in Ethernet and broadcast environments in which the election of DR/BDR takes place.

Scenario 3-4 illustrates the behavior of OSPF in an NBMA environment.