Module 3 - OSPF Network Types pot

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BSCI Module 3 Lesson 4 - OSPF Network Types
OSPF
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Objective
 OSPF Network Types
 Adjacency Behavior for a Point-to-Point Link
 Adjacency Behavior for a Broadcast Link
 Selecting the DR and BDR
 Adjacency Behavior for a NBMA Network
 DR Election in NBMA Topology
 OSPF Over Frame Relay
 OSPF over NBMA Topology Modes
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Different Network Types and OSPF
 There are three distinct classifications of physical network media that
OSPF can differentiate between. Each of these network types requires a
slightly different configuration to optimize the performance of OSPF.
 Cisco IOS Software allows five main network types to be configured, as
displayed in Table 5-5.
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OSPF Network Types
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Configuring the Network Type
 OSPF is highly adaptable. Consider its capability to accommodate either
broadcast or nonbroadcast multiaccess (NBMA) network types. OSPF
responds accordingly by altering its operation to reflect the configuration
that you have given it.
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Broadcast Networks
 Set an interface to operate in broadcast network mode when the network
is a fully meshed.
 When this command is applied to an interface, OSPF elects a DR/BDR,
builds an adjacency with the DR, and builds neighbor relationships with all
other OSPF routers in the network.
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Nonbroadcast Networks
 When configuring an interface as nonbroadcast, OSPF cannot perform
multicasting on that link.
 Lack of multicast functionality impacts OSPF’s operation because OSPF
Hellos cannot be properly transmitted.
 Physical Frame Relay interfaces and multipoint subinterfaces are

recognized as nonbroadcast by default. The following alternatives allow
you to make OSPF operate when a nonbroadcast type network is
encountered:
–Change the network type of OSPF. This is the easiest change
but is not possible in every instance.
–Manually define the neighbors. This forces adjacencies that
are created by sending unicast Hello packets.
–Alter the behavior of the Layer 2 protocol.
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Nonbroadcast Networks
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Point-to-Multipoint Networks
 OSPF point-to-multipoint interfaces are defined as numbered point-to-
point subinterfaces, with the router having one or more OSPF neighbors.
 An OSPF point-to-multipoint network has the following benefits compared
to nonbroadcast multiaccess and point-to-point networks:
–Point-to-multipoint is easier to configure because it requires no
configuration of neighbor commands. Point-to-multipoint
connections consume only one IP subnet and require no
designated router election process.
–Point-to-multipoint has a fully or partial meshed topology.
–Point-to-multipoint can be more reliable in fully meshed
networks because it maintains connectivity in case of virtual
circuit failure.

–When you configure the OSPF network type as point-to-
multipoint, routing between two routers not directly connected
goes through the router (hub) that has virtual circuits to both
routers (spokes).
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Point-to-Multipoint Networks
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Point-to-Point Networks
 When you configure an interface as an OSPF point-to-point network type,
this command understands that the only other OSPF-speaking router on
that interface is directly connected.
 As previously mentioned, the following methods can be used to get OSPF
to operate in such a configuration:
–Use point-to-point subinterfaces
–Use a physical interface and an OSPF point-to-point network type
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Adjacency Behavior for a Point-to-Point Link
 Hello time 10s, Dead time 40
 Neighboring routers become adjacent whenever they
can communicate directly
 Usually, the IP source address of an OSPF packet is

set to the address of the outgoing interface on the
router.
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Adjacency Behavior for a Broadcast Link
 Reduce routing update traffic
 Manage link-state synchronization
 Note:
After a DR and BDR have been selected, any router added to the
network establishes adjacencies with the DR and BDR only.
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Selecting the DR and BDR
 It is more efficient to have one router represent the
other routers to a new router rather than all the routers
pass the same information.
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Setting Priority for DR election
 The BDR uses the wait timer to determine whether the
DR is out of service.
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Adjacency Behavior for a NBMA Network
 If the NBMA topology is not fully meshed, a broadcast
or multicast sent by one router will not reach all the
other routers.
 To implement broadcasting or multicasting router
replicates the packets sends them individually on each
permanent virtual circuit (PVC)
Hello Time 30s; Dead Time 120s
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DR Election in NBMA Topology
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OSPF Over Frame Relay
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OSPF over NBMA Topology Modes of
Operation
 RFC 2328-compliant modes are as follows:
–Nonbroadcast (NBMA)
–Point-to-multipoint
 Additional modes from Cisco are as follows:
–Point-to-multipoint nonbroadcast

–Broadcast
–Point-to-point
ip ospf network [{broadcast | non-broadcast | point-to-
multipoint [non-broadcast] | point-to-point}]
 This interface command defines OSPF network type.
Router(config-if)#
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Activity
 3.4.1 Drag and Drop: OSPF Network Types
–Upon completion of this activity, the student will be able to
identify the different OSPF network types.
 3.4.4 Checkbox: DR and BDR Election
–Upon completion of this activity, the student will be able to
identify the process of DR and BDR election.
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Q and A
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