OSPF Across Nonbroadcast Multiaccess Networks

A nonbroadcast multiaccess (NBMA) network has certain characteristics. The main ones are identified in the name of the technology: It is a network that cannot carry broadcast traffic but that has multiple destinations. Examples of NBMA networks include Frame Relay, X.25, and ATM. The Routing exam concentrates on the use of OSPF across Frame Relay.

The crux of the problem is how OSPF operates using multicast traffic to exchange network information and to create adjacencies to synchronize databases across this WAN cloud without using the multicast addresses.

The solution to the problem varies, depending on the technology involved and the network design. The modes available fall into two technologies, within which there are additional options. The two technologies are point-to-point and NBMA.

The NBMA technology is then subdivided into two categories, under which different configuration options are available. These two categories are the RFC-compliant solution and the Cisco-offered solution:

• RFC-compliant—The RFC-compliant category offers a standard's solution, which is independent of the vendor platform. The configuration options are these:

— Point-to-multipoint

• Cisco-specific—These configuration options are proprietary to Cisco and include these:

— Point-to-multipoint nonbroadcast

The option selected depends on the network topology that is in use. The OSPF technology is separate from the physical configuration, and the choice of implementation is based on the design topology.

The Frame Relay topologies include these:

• Full mesh—Every router is connected to every other router. This solution provides redundancy, and it may allow load sharing. This is the most expensive solution.

• Partial mesh—Some routers are connected directly; others are accessed through another router.

• Star, or hub and spoke—One router acts as the connection to every other router. This is the least expensive solution because it requires the fewest number of permanent virtual circuits (PVCs). Here a single interface is used to connect to multiple destinations.

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