Content

| 6.4 |  | Configuring OSPF Over NBMA |
| 6.4.3 |  | Partial-mesh Frame Relay |

Because a full-mesh topology is costly, many organizations implement a partial-mesh topology instead. A partial-mesh topology is any configuration in which at least one router maintains multiple connections to other routers, without being fully meshed. The most cost-effective partial-mesh topology is a hub-and-spoke topology. This is where a single router, the hub, connects to multiple spoke routers.

The hub-and-spoke topology is a cost effective WAN solution that introduces a single point of failure, the hub router. Organizations typically use Frame Relay because it is inexpensive, not because it is fault tolerant. Since dedicated leased lines typically carry mission critical data, an economical Frame Relay topology, such as hub-and-spoke, makes sense.

Unfortunately, the neighbor command that worked with a full-mesh topology does not work as well with the hub-and-spoke topology. The hub router in Figure sees all the spoke routers and can send routing information to them using the neighbor command, but the spoke routers can send Hellos only to the hub.

The DR or BDR election will be held, but only the hub router sees all of the candidates. Because the hub router must act as the DR for this OSPF network to function properly, configure an OSPF interface priority of zero (0) on all the spoke routers. Recall that a priority of zero (0) makes it impossible for a router to be elected as a DR or a BDR for a network.

A second approach to dealing with this topology is to avoid the DR and BDR issue altogether by breaking the network into point-to-point connections. Point-to-point networks will not elect a DR or a BDR.

Although they make OSPF configuration straightforward, point-to-point networks have major drawbacks when used with a hub-and-spoke topology. Subnets must be allocated for each link. This can lead to WAN addressing that is complex and difficult to manage. The WAN addressing issue can be avoided by using IP unnumbered, but many organizations have WAN-management policies that prevent using this feature. Are there any possible alternatives to a point-to-point configuration? Fortunately, the Cisco IOS offers a relatively new alternative. A hub-and-spoke physical topology can be manually configured as a point-to-multipoint network type, as described in the following section.