4.4 OSPF Over Non-Broadcast Networks
4.4.4 Point-to-Multipoint mode
Point-to-Multipoint networks have been designed to work simply and naturally when faced with partial mesh connectivity. In Point-to-Multipoint mode, OSPF treats all router-to-router connections on the non-broadcast network as if they were point-to-point links. No Designated Router is elected for the network, nor is there an LSA generated for the network. It may be necessary to configure the set of neighbors that are directly reachable over the Point-to-Multipoint network. Each neighbor is identified by its IP address on the Point-to-Multipoint network. Because no Designated Routers are elected on Point-to-Multipoint networks, the Designated Router eligibility of configured neighbors is undefined.

Alternatively, neighbors on Point-to-Multipoint networks may be dynamically discovered by lower-level protocols such as Inverse ARP. In contrast to NBMA networks, Point-to-Multipoint networks have the following properties:

  1. Adjacencies are established between all neighboring routers. There is no Designated Router or Backup Designated Router 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, nor for neighbors on Point-to-MultiPoint networks.
  2. When originating a router-LSA, Point-to-MultiPoint interface is reported as a collection of "point-to-point links" to all of the interface's adjacent neighbors, together with a single stub link advertising the interface's IP address with a cost of 0.
  3. When flooding out a non-broadcast interface (when either in NBMA or Point-to-MultiPoint mode) the Link State Update or Link State Acknowledgment packet must be replicated in order to be sent to each of the interface's neighbors.
Lab Activity
  In this lab, you will learn how to configure OSPF to function correctly in a non-broadcast environment.