Content

| 6.4 |  | Configuring OSPF Over NBMA |
| 6.4.1 |  | NBMA overview |

This module 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 as shown in Figure . Therefore, NBMA 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. RTA, RTB, and RTC belong to the same IP subnetwork and will attempt to elect a DR and a BDR. However, these routers cannot hold a valid election if they cannot 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 depend on the current NBMA network topology.

| Interactive Media Activity Drag and Drop: OSPF Network Types 2 Upon completion of this activity, the student will be able identify the different fields in a routing table entry.

| Web Links Configuring OSPF http://www.cisco.com/en/US/products/ sw/iosswrel/ps1828/products_configuration_ guide_chapter09186a00800ca56c.html