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| 6.4 |  | Configuring OSPF Over NBMA |
| 6.4.4 |  | Point-to-Multipoint OSPF |

In a point-to-multipoint network, a hub router is directly connected to multiple spoke routers, but all the WAN interfaces are addressed on the same subnet.

This logical topology was seen earlier in the module. However, it was also learned that OSPF does not work properly as an NBMA OSPF network type. By manually changing the OSPF network type to point-to-multipoint, this logical topology can then work. Routing between RTA and RTC will go through the router that has virtual circuits to both routers, RTB. Notice that it is not necessary to configure neighbors when using this feature. Inverse ARP will discover them.

Point-to-multipoint networks have the following properties:

- Adjacencies are established between all neighboring routers. There is no DR or BDR for a point-to-multipoint network. No network LSA is originated for point-to-multipoint networks. Router priority is not configured for point-to-multipoint interfaces or for neighbors on point-to-multipoint networks.
- When originating a router LSA, the point-to-multipoint interface is reported as a collection of point-to-point links to all the adjacent neighbors on the interface. This is together with a single stub link advertising the IP address of the interface with a cost of 0.

When flooding out a nonbroadcast interface, the LSU or LSAck packet must be replicated to be sent to each of the neighbors on the interface.

To configure point-to-multipoint, manually override the detected OSPF network type with the following syntax:

> Router(config-if)#ip ospf network point-to-multipoint

Also configure the interface with a frame-relay map ip command, as in the following syntax:

> Router(config-if)#frame-relay map ip address dlci broadcast

The broadcast keyword permits the router to send broadcasts by way of the specified DLCI to the mapped neighbor or neighbors. If applying the point-to-multipoint configuration to the example network shown in Figure , two separate frame-relay map statements would have to be configured on the hub router, RTB. Partial configurations for each router are shown in Figure .

In a point-to-multipoint configuration, OSPF treats all router-to-router connections on the nonbroadcast network as if they were point-to-point links. No DR is elected for the network. Neighbors can be manually specified using the neighbor command or can be dynamically discovered using Inverse ARP.

Ultimately, point-to-multipoint OSPF offers efficient operation without administrative complexity.