Content

| 6.1 |  | OSPF Overview |
| 6.1.4 |  | OSPF network types |

Adjacency is required to allow for OSPF routers to share routing information, a router will try to become adjacent to at least one other router on each IP network to which it is connected. Some routers may try to become adjacent to all their neighbor routers, and others may try with only one or two. OSPF routers determine which routers to become adjacent to based on what type of network connects them.

OSPF interfaces automatically recognize three types of networks: broadcast multiaccess, nonbroadcast multiaccess (NBMA), and point-to-point networks. An administrator can configure a fourth network type, a point-to-multipoint network.

The type of network dictates how OSPF routers should relate to each other. An administrator may have to override the detected network type in order for OSPF to operate properly.

Some networks are defined as multiaccess because it cannot be predicted just how many routers are connected to them. There may be one, two, or more routers. A campus that uses a switched Ethernet core may have half a dozen routers connected to the same backbone network. A school district might have 10, 12, or 25 remote-site routers connected by way of Frame Relay PVCs to the same IP subnet.

A significant number of routers can exist on a multiaccess network. The designers of OSPF developed a system to avoid the overhead that would be created if every router established full adjacency with every other router. This system restricts who can become adjacent to whom by employing the services of one of the following:

- Designated router (DR) – For every multiaccess IP network, one router will be elected the DR. The DR has two main functions. The first function is to become adjacent to all other routers on the network, and second, is to act as a spokesperson for the network. As spokesperson, the DR will send network LSAs for all other IP networks to every other router. Because the DR becomes adjacent to all other routers on the IP network, it is the focal point for collecting routing information (LSAs).
- Backup designated router (BDR) – The DR could represent a single point of failure, so a second router is elected as the BDR to provide fault tolerance. Therefore, the BDR must also become adjacent to all routers on the network and must serve as a second focal point for LSAs. However, unlike the DR, the BDR is not responsible for updating the other routers or sending network LSAs. Instead, the BDR keeps a timer on the update activity of the DR to ensure that it is operational. If the BDR does not detect activity from the DR before the timer expires, the BDR takes over the role of DR and a new BDR is elected.

On point-to-point networks, only two nodes exist. Therefore, a focal point for routing information is not needed. No DR or BDR is elected. Both routers become fully adjacent to one and other.

| Interactive Media Activity Drag and Drop: OSPF Network Type 1 Upon completion of this activity, the student will be able to identify the different OSPF Network types.