168 Chapter 6: OSPF Network Topologies
Foundation Topics
OSPF Network Topology Options
OSPF is capable of routing over every type of data link, but OSPF makes assumptions that do not
hold true for all topologies. OSPF assumes that—within a subnet—all routers can communicate
directly using multicasts and that no router is uniquely positioned in the topology. 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).
The aforementioned assumptions do not hold for nonbroadcast multiaccess (NBMA)
environments. In a Frame Relay network, for example, multicasts and broadcasts are not
supported on “NBMA” OSPF-network-type interfaces on Cisco routers. The following are the
OSPF network types available on Cisco router interfaces:
Broadcast multiaccess
Point-to-multipoint (default is point-to-multipoint broadcast; nonbroadcast option is
Nonbroadcast multiaccess (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. This chapter describes several strategies for dealing with neighbor discovery and
communication in an NBMA topology.
Any-to-any communication within a subnet cannot be assumed in NBMA. DRs need to be able to
communicate with all other devices, which leads to design considerations. This chapter discusses
the strategies available for overcoming this limitation.
Multipoint interfaces on Cisco routers automatically start in nonbroadcast mode, but there
are times when this needs to be manually adjusted. Each of the four OSPF network types has
its own advantages and disadvantages, as described in this section. After explaining the
NOTE Neither of these issues applies to LANs or to point-to-point links. Ethernet links are
automatically recognized as broadcast links. Point-to-point links, such as DS3, are also
automatically recognized.
OSPF Network Topology Options 169
differences between these OSPF network types and the use of subinterfaces, this section describes
how to select an OSPF design.
Understanding the Differences Between OSPF Network Types
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. The following sections compare the properties of each OSPF network type.
Designated Routers and Topology Support
DRs minimize topology traffic, but a DR works under the assumption that it is in contact with all
devices. 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. If the topology is not a full mesh, then
the DR should be manually selected, using priorities, to be a router with permanent virtual circuits
(PVCs) connecting it to all other routers. For example, in a hub-and-spoke topology, the DR should
be the hub and all spokes should have a priority of zero (there will not be a separate BDR).
The point-to-multipoint OSPF network type does not involve a DR election; this OSPF network
type is less efficient in a full mesh, but fine with a partial mesh. It is more tolerant of network
changes than the broadcast option. The point-to-multipoint OSPF network type also has a
Cisco-specific option called point-to-multipoint nonbroadcast.
The point-to-point OSPF network type also does not involve a DR election.
OSPF network types can be described as either RFC-compliant or Cisco proprietary, as follows:
RFC-compliant—RFC-compliance offers a vendor-neutral routing platform. There are two
nonbroadcast OSPF network types (RFC 2328):
Cisco-specific—Of the broadcast, point-to-multipoint (broadcast), point-to-multipoint
nonbroadcast, point-to-point, and NBMA OSPF network type options on a Cisco router that
can be used in nonbroadcast environment, three of these are Cisco proprietary:
Point-to-multipoint nonbroadcast

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