80 Chapter 2: Introduction to OSPF

The following list explains the characteristics of the OSPF network types as illustrated in Figure 2-10:

• Broadcast—A network type that connects two or more OSPF routers over a broadcast media such as Ethernet or FDDI. Neighbor relationships are developed using OSPF Hellos; from that, a designated router (DR) and backup designated router (BDR) are formed via adjacencies to them.

NOTE The next section fully explains OSPF neighbor relationships, adjacencies, and DRs.

• Nonbroadcast multiaccess (NBMA) —NBMA networks do not allow broadcasts by default; examples include frame relay, ATM, or X.25 networks. NBMA networks also have the potential for multiple adjacencies, but because they do not all send broadcasts, they cannot guarantee the proper relationships will form.

• Point-to-multipoint—A method of configuring NBMA networks that allows OSPF to operate as if the routers were connected via point-to-point links instead of via an NBMA network. There are no DRs or BDRs in this configuration because each link is treated as a point-to-point link.

• Point-to-point—A single circuit that connects two OSPF routers, which allows a single-neighbor relationship to be built. Some examples of this are leased lines running Point-to-Point Protocol (PPP) or High-Level Data Link Control (HDLC). There are no DRs or BDRs in this network type.

Configuring the OSPF network type is done at the interface level of a router, as demonstrated in Example 2-3.

Example 2-3 Configuring the OSPF Network Type

HAL9000(config-if)#ip ospf network ?

broadcast Specify OSPF broadcast multi-access network non-broadcast Specify OSPF NBMA network point-to-multipoint Specify OSPF point-to-multipoint network point-to-point Specify OSPF point-to-point network

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