OSPF Network Topologies

Figure 5-2 A Broadcast Multiaccess Network


• Point-to-point—This technology is used where there is one other system directly connected to the transmitting or receiving router. A typical example of this is a serial line. OSPF has no need of a designated or backup designated router in this scenario. Network traffic uses the multicast address for OSPF AllSPFRouters, Figure 5-3 illustrates a point to point network.

Figure 5-3 Point-to-Point Network

• Point-to-multipoint—This is a single interface that connects to many destinations. The underlying network treats the network as a series of point-to-point circuits. It replicates LSA packets for each circuit. The addressing of network traffic is multicast. There is no DR or BDR election. This technology uses one IP subnet.

Physically, some point-to-multipoint networks cannot support multicast or broadcast traffic. In these cases, special configuration is required. The configuration and considerations of an NBMA network are considered later in this chapter. Figure 5-4 illustrates a point-to-multipoint network.

Figure 5-4 Point-to-Multipoint Network

Figure 5-4 Point-to-Multipoint Network

• Nonbroadcast multiaccess (NBMA)—This physically resembles a point-to-point line, but in fact, many destinations are possible. WAN clouds, including X.25 and Frame Relay, are examples of this technology. NBMA uses a fully meshed or partially meshed network. OSPF sees it as a broadcast network, and it will be represented by one IP subnet.

This technology requires manual configuration of the neighbors and the DR and BDR selection. The configuration options have increased with the different versions of Cisco IOS.

DR and BDR routers are elected, and the DR will generate an LSA for the network. The DR and BDR must be directly connected to their neighbors. All network traffic sent between neighbors will be replicated for each physical circuit using unicast addresses because multicast and broadcast addresses are not understood. Figure 55 illustrates an NBMA network.

• Virtual links—This is a virtual connection to a remote area that does not have any connections to the backbone (Area 0). Although OSPF treats this link as a direct, singlehop connection to the backbone area, it is a virtual connection that tunnels through the network. The OSPF network traffic is sent in unicast datagrams across these links.

The WAN topologies will be discussed in detail later in the chapter, in the section, "Configuring OSPF over an NBMA Topology." Suffice it to say that the method by which the routers in an OSPF network find one another and exchange information depends on the physical characteristics of the network.

Although the routers running OSPF transmit a small packet called the hello packet to establish neighbor relations, it serves other functions. The various fields in the hello packet have specific responsibilities. These are shown in the Table 5-3.

Figure 5-5 A Nonbroadcast Multiaccess (NBMA) Network

Figure 5-5 A Nonbroadcast Multiaccess (NBMA) Network

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