4.4 Configuring OSPF Over NBMA
4.4.1
NBMA overview
This chapter has focused on two types of OSPF networks in detail: broadcast multiaccess and point-to-point networks. Even if there is only one router, broadcast multiaccess networks elect a DR and a BDR to serve as focal points for routing information. In contrast, point-to-point OSPF networks do not elect a DR because they can never include more than two nodes.

Another type of OSPF network, Nonbroadcast Multiaccess (NBMA), can include more than two nodes and therefore will try to elect a DR and a BDR. Common NBMA implementations include Frame Relay, X.25, and SMDS. NBMA networks follow rules at Layer 2 that prevent the delivery of broadcasts and multicasts. Figure summarizes the OSPF network types.

NBMA networks can create problems with OSPF operation, specifically with the exchange of multicast hello packets. In the example shown in Figure , RTA, RTB, and RTC belong to the same IP subnetwork and will attempt to elect a DR and a BDR. However, these routers can not hold a valid election if they can not receive multicast hellos from every other router on the network. Without administrative intervention, a strange election takes place. As far as RTA is concerned, RTC is not participating. Likewise, RTC goes through the election process oblivious to RTA. This botched election can lead to problems if the central router, RTB, is not elected the DR.

The Cisco IOS offers several options for configuring OSPF to overcome NBMA limitations, including the OSPF neighbor command, point-to-point subinterfaces, and point-to-multipoint configuration. The solutions that are available to you depend on your NBMA network topology.