Multiaccess topologies, such as those found in NBMA and broadcast OSPF network types, rely on DRs and so are best applied to full-mesh topologies. Designated Routers and Topology SupportĭRs minimize topology traffic, but a DR works under the assumption that it is in contact with all devices. The following sections compare the properties of each OSPF network type. The key differences between the four OSPF network types revolve around the use of DRs, support for partial mesh Frame Relay topologies, support for standards, neighbor recognition, and timers. Understanding the Differences Between OSPF Network Types After explaining the differences between these OSPF network types and the use of subinterfaces, this section describes how to select an OSPF design. Each of the four OSPF network types has its own advantages and disadvantages, as described in this section. Multipoint interfaces on Cisco routers automatically start in nonbroadcast mode, but there are times when this needs to be manually adjusted. Point-to-point links, such as DS3, are also automatically recognized. Ethernet links are automatically recognized as broadcast links. Neither of these issues applies to LANs or to point-to-point links. This chapter discusses the strategies available for overcoming this limitation. DRs need to be able to communicate with all other devices, which leads to design considerations. This chapter describes several strategies for dealing with neighbor discovery and communication in an NBMA topology.Īny-to-any communication within a subnet cannot be assumed in NBMA. To account for the lack of multicast and broadcast support inherent in NBMA OSPF-network-type interfaces on Cisco routers, multicasts are simulated by replicating an advertisement to each neighbor. Point-to-multipoint (default is point-to-multipoint broadcast nonbroadcast option is available) The following are the OSPF network types available on Cisco router interfaces: In a Frame Relay network, for example, multicasts and broadcasts are not supported on "NBMA" OSPF-network-type interfaces on Cisco routers. The aforementioned assumptions do not hold for nonbroadcast multiaccess (NBMA) environments. Both assumptions are fine for Ethernet: If five routers are attached to a switch, a multicast from one reaches the other four and each would be fine as a designated router (DR). OSPF assumes that-within a subnet-all routers can communicate directly using multicasts and that no router is uniquely positioned in the topology. OSPF is capable of routing over every type of data link, but OSPF makes assumptions that do not hold true for all topologies.
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