4.4 OSPF Over Non-Broadcast Networks
4.4.1
Non-Broadcast Multiple Access (NBMA) overview
NBMA networks are those networks that support many (more than two) routers, but have no broadcast capability. Neighboring routers are maintained on these nets using OSPF's Hello Protocol. However, due to the lack of broadcast capability, some configuration information may be necessary to aid in the discovery of neighbors. On non-broadcast networks, OSPF protocol packets that are normally multicast need to be sent to each neighboring router. An X.25 Public Data Network (PDN) is an example of a non-broadcast network.

Note the following:

NBMA networks, such as the one in Figure , pose a special problem for OSPF and DR election. On a Cisco router, these networks can be configured to act as a single broadcast interface with multiple connections.

Because Router A is using a single multipoint interface (a Frame Relay interface using inverse ARP or frame-map configurations to separate the traffic between the permanent virtual circuits [PVC's]) when Router A broadcasts a packet, all the other routers receive it. But when Routers B or F broadcast a packet, the only router that receives the packet is Router A.

Because all the routers connected to this multi-access network assume it is a single broadcast domain, they will attempt, unsuccessfully, to elect a BDR and DR. Assuming that all routers are connected to the link at the same time, the following scenario will occur:

  • Routers A and B will elect Router A as the DR and Router B as the BDR.
  • Routers A and F will elect Router F as the DR and Router A as the BDR.
  • Router B will not receive Router F's hellos.
  • Router F will not receive Router B's hellos.

Essentially, this is broken; there is no way to determine what the final outcome will be. It may actually work for some time until a link flaps or one of the routers on the network goes down.

There are 3 possible solutions to this problem:

  • Set all remote sites to OSPF priority 0, and the hub or core router to anything else.
  • Use point-to-point subinterfaces
  • Configure the network as a point-to-multipoint network type

The first solution - configuring the OSPF router priorities - was the only solution for some time (before the availability of point-to-point subinterfaces or network type point-to-multipoint). Some network administrators, however, configure the remote routers with a low priority rather than a priority of 0, which works but can still cause problems because the BDR status will be in question. It is best to simply configure the remote routers to be ineligible to become a DR or BDR. 

The second solution - using point-to-point subinterfaces - has been available for some time now and has many advantages. It has one disadvantage that many administrators don't like though: A separate network address must be used for each serial link. If a network has a lot of remote sites connected to distribution or access layer routers in this fashion, this can become a major administrative nightmare.

The final solution - network type point-to-multipoint - is a recent development. Instead of the hub router treating the NBMA network as a broadcast domain, it treats each PVC as a point-to-point link, building full adjacencies with each router. This technique is effective, but it results in the creation of host routes for each remote router on the NBMA network.