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| 4.6 |  | PPP and Layer 2 Considerations for Routed and Routing Protocols |
| 4.6.5 |  | OSPF in an NBMA environment |

Unless special consideration is given to the operation of OSPF in a Non-Broadcast Multi-Access (NBMA) environment such as Frame Relay, routing problems will occur. This is because OSPF expects to be able to multicast all routers within a subnet, or broadcast domain. In a Frame Relay hub and spoke topology this will not be possible. Consequently, some routers and routes will be inaccessible.

The problem can be solved by forcing RTB to become the designated router. This is achieved by configuring an OSPF interface priority of 0 on all the spoke routers. Recall that a priority of 0 makes it impossible for a router to be elected as DR or a BDR for a network. As long as RTB can exchange hellos with RTA and RTC it can maintain a complete view of the network and pass this routing information to both RTA and RTC.

Alternatively, subinterfaces could be used on the hub router making each link a point to point link. Recall that no DR election occurs in a point to point configuration. The downside to this technique is the creation of a new subnet or network.

Still another technique to manage OSPF over hub and spoke NBMA networks is to explicitly configure OSPF as a Point-to-multipoint network. 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. 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, manually override the detected OSPF network type with the following syntax:

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

The interface should also be configured 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 the point-to-multipoint configuration is applied to the example network, two separate frame-relay map statements would have to be configured on the hub router, RTB.