4.4 Configuring OSPF Over NBMA
4.4.4 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.

You saw this logical topology earlier in the chapter, but you learned that OSPF does not work properly as an NBMA OSPF network type. By manually changing the OSPF network type to point-to-multipoint, you can make this logical topology work. Routing between RTA and RTC will go through the router that has virtual circuits to both routers, RTB. Note 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 interface's adjacent neighbors, together with a single stub link advertising the interface's IP address 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 interface's neighbors.

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

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

You should 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 via the specified DLCI to the mapped neighbor or neighbors. If you apply the point-to-multipoint configuration to the example network , you would have to configure two separate frame-relay map statements 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.