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| 6.4 |  | Configuring OSPF Over NBMA |
| 6.4.2 |  | Full-mesh Frame Relay |

The different NBMA topologies must be understood before selecting an OSPF configuration strategy for a Frame Relay network, or legacy X.25 network. Fundamentally, two possible physical topologies exist for Frame Relay networks.

- Full-mesh topology
- Partial-mesh topology, including the hub-and-spoke topology

The following sections describe how to configure OSPF in both full-mesh and partial-mesh Frame Relay networks.

Full-Mesh Frame Relay
Organizations use Frame Relay primarily because it supports more than one logical connection over a single interface. This makes it an affordable and flexible choice for WAN links. A full-mesh topology takes advantage of the capabilities Frame Relay has to support multiple permanent virtual circuits (PVCs) on a single serial interface. In a full-mesh topology, every router has PVCs to every other router.

For OSPF to work properly over a multiaccess full-mesh topology that does not support broadcasts, each OSPF neighbor addresses must be manually entered on each router, one at a time. The OSPF neighbor command tells a router about the IP addresses of its neighbors so that it can exchange routing information without multicasts. The following example illustrates how the neighbor command is used:

> RTA(config)#router ospf 1
> RTA(config-router)#network 3.1.1.0 0.0.0.255 area 0
> RTA(config-router)#neighbor 3.1.1.2
> RTA(config-router)#neighbor 3.1.1.3

Specifying the neighbors for each router is not the only option to make OSPF work in this type of environment. The following section explains how configuring subinterfaces can eliminate the need for the neighbor command.

Configuring Subinterfaces to Create Point-to-Point Networks
The IOS subinterface feature can be used to break up a multiaccess network into a collection of point-to-point networks.

In Figure , a different IP subnet is assigned to each PVC. OSPF automatically recognizes this configuration as point-to-point, not NBMA, even with Frame Relay configured on the interfaces. Recall that OSPF point-to-point networks do not elect a DR. Instead, the Frame Relay router uses Inverse ARP or a Frame Relay map to obtain the link partner's address so that routing information can be exchanged.

A full-mesh topology offers numerous advantages, including maximum fault tolerance. Unfortunately, full-mesh topologies can get expensive because each PVC must be leased from a provider. An organization would have to lease 45 PVCs to support just ten fully meshed routers. If subinterfaces are used to create point-to-point networks, then the 45 IP subnets must also be allocated and managed, which is an additional expense.