network, the nonbroadcast nature of the network can create reachability issues. NBMA networks can support more than two routers but have no broadcast capability. For example, if the NBMA topology is not fully meshed, a broadcast or multicast sent by one router will not reach all the other routers. Frame Relay, ATM, and X.25 are examples of NBMA networks. To implement broadcasting or multicasting on an NBMA network, the router replicates the packets to be broadcast or multicast and sends them individually on each permanent virtual circuit (PVC) to all destinations. This process is CPU and bandwidth intensive. The default OSPF hello and dead intervals on NBMA interfaces are 30 seconds and 120 seconds, respectively.

Content 3.4 OSPF Network Types 3.4.6 DR Election in NBMA Topology OSPF considers that the NBMA environment functions in a way similar to other broadcast media such as Ethernet. However, NBMA clouds are usually built in hub-and-spoke topologies, using PVCs or switched virtual circuits (SVCs). A hub-and-spoke topology means that the NBMA network is only a partial mesh. In these cases, the physical topology does not provide the multiaccess capability that OSPF relies on.Electing a DR is an issue in NBMA topologies because the DR and BDR need to have full physical connectivity with all routers in the NBMA network. The DR and BDR also need to have a list of all the other routers so that they can establish adjacencies. OSPF cannot automatically build adjacencies with neighboring routers over NBMA interfaces.

Content 3.4 OSPF Network Types 3.4.7 OSPF Over Frame Relay There are several OSPF configuration choices for a Frame Relay network, depending on the network topology. With Frame Relay, remote sites interconnect in a variety of ways. By default, interfaces that support Frame Relay are multipoint connection types. The following are types of Frame Relay topologies:

Hub-and-Spoke: A hub-and-spoke topology, also known as a star configuration, is the most common Frame Relay network topology. Remote sites connect to a central site that generally provides a service or application. It is the least expensive topology because it requires the least PVCs. The central router provides a multipoint connection, because it typically uses a single interface to interconnect multiple PVCs.
Full-mesh: All routers have virtual circuits to all other destinations. This topology, although costly, provides direct connections from each site to all other sites and allows for redundancy. As the number of nodes increases, it becomes increasingly expensive.

To figure out how many virtual circuits are needed to implement a fully meshed topology, use the formula n (n – 1) / 2, where n is the number of nodes in the network.

Partial-mesh: Not all sites have direct access to a central site. This method reduces the cost of implementing a full-mesh topology.

Content 3.4 OSPF Network Types 3.4.8 OSPF over NBMA Topology Modes As described in RFC 2328, OSPF runs in one of the following two modes in NBMA topologies:

Nonbroadcast: Simulates the operation of OSPF in broadcast networks. Neighbors must be manually configured, and DR and BDR election is required. This configuration is typically used with fully meshed networks.
Point-to-multipoint: Treats a nonbroadcast network as a collection of point-to-point links. The routers automatically identify their neighboring routers but do not elect a DR and BDR. This configuration is typically used with partially meshed networks.

The operation mode determines the way the hello protocol and flooding work over the nonbroadcast network. The main advantage of point-to-multipoint is that it requires less manual configuration, and the main advantage of nonbroadcast is that there is less overhead traffic. Cisco has defined the following additional modes:

Point-to-multipoint nonbroadcast
Broadcast
Point-to-point

Content 3.5 Implementing OSPF in an NBMA Network 3.5.1 Configuring the OSPF Network Type You can use the ip ospf network interface command to select the OSPF network type for NBMA networks. Figure describes the options. Figure displays a sample configuration of a Frame Relay router in a full-mesh topology that uses broadcast mode. Broadcast mode is a workaround for statically listing all existing neighboring routers. The interface behaves as though the router is connected to a LAN. DR and BDR election is still performed; therefore, take special care to ensure either a full-mesh topology or a deterministic election of the DR using the interface priority is used.

Content 3.5 Implementing OSPF in an NBMA Network 3.5.2 Configuring OSPF over Frame Relay In nonbroadcast mode, OSPF emulates operation over a broadcast network. A DR and BDR are elected for the NBMA network, and the DR originates an LSA for the network. In this environment, the routers are usually fully meshed to facilitate the establishment of adjacencies among the routers. If the routers are not fully meshed, you should select the DR and BDR manually to ensure that the selected DR and BDR have full connectivity to all other neighboring routers. Neighboring routers are statically defined to start the DR and BDR election process. When using nonbroadcast mode, all routers are on one IP subnet. For flooding over a nonbroadcast interface, the LSU packet must be replicated for each PVC. The updates are sent to each of the neighboring routers defined in the neighbor table on the interface. When there are few neighbors in the network, nonbroadcast mode is the most efficient way to run OSPF over NBMA networks because it has less overhead than point-to-multipoint mode. Frame Relay, ATM, and X.25 networks default to OSPF nonbroadcast mode.

Content 3.5 Implementing OSPF in an NBMA Network 3.5.3 Using the neighbor Command You can use the OSPF neighbor command to statically define adjacent relationships in NBMA networks using nonbroadcast mode. Figure displays the command options. Figure illustrates an example of statically defining adjacencies. All three routers are using the default nonbroadcast mode on their Frame Relay interfaces; therefore, each one must manually configure its neighboring routers. The priority should be set to 0 for routers B and C. This configuration ensures that router A becomes the DR, because only router A has full connectivity to the other two routers. A BDR will not be elected in this case. In an NBMA network, neighbor statements are required only on the DR and BDR. In a hub-and-spoke topology, neighbor statements must be used on the hub, which must be configured to become the DR. Neighbor statements are not mandatory on the spoke routers. In a full-mesh NBMA topology, you may need neighbor statements on all routers, unless you have statically configured the DR and BDR using the priority option.

Content 3.5 Implementing OSPF in an NBMA Network 3.5.4 The show ip ospf neighbor Command Router A in Figure has a serial Frame Relay NBMA interface and a Fast Ethernet interface. To view neighbor adjacency details, use the show ip ospf neighbor [type number] [neighbor-id] [detail] command. The serial 0/0/0 interface is connected to Router B and Router C. The command output displays a state of FULL/DROTHER. FULL means that router A is fully adjacent with its neighbors. DROTHER indicates that the neighboring routers are not a DR or BDR. This is because router A is the DR, and there is no BDR in this network. The FastEthernet 0/0 interface of router A is connected to router D. The command output displays a state of FULL/BDR. Again, router A is fully adjacent with router D, and that router D is a BDR. According to the topology diagram, there are no other routers connected on this multiaccess broadcast link;