CCNP BSCI Notes – OSPF Network Topologies


Network Topology Options

OSPF assumes a subnet is broadcast-capable by default.

OSPF Network Types

Broadcast multiaccess


Point-to-multipoint broadcast

Point-to-multipoint nonbroadcast

Nonbroadcast multiaccess (NBMA)

NBMA and point-to-multipoint are standards-compliant (RFC 2328), whereas point-to-multipoint nonbroadcast, broadcast, and point-to-point implementations are Cisco proprietary.

NBMA networks utilize DRs like broadcast networks, however neighbors must be manually defined instead of being automatically discovered.


Configuring OSPF in a Nonbroadcast Environment

Nonbroadcast Network

Because NBMA is the default network type for a nonbroadcast interface, the only necessary

configuration is to define neighbors.

DR priorities should be specified to ensure only candidates positioned well in the topology are elected DR and BDR.


priority – This can be used to specify a higher priority than what has been configured on the neighbor (but not lower)

poll interval – The rate at which hellos are sent to inactive neighbors (default 120 seconds) cost – Cost to reach the neighbor

NBMA configuration:


Point-to-multipoint Network

Point-to-multipoint automatically establishes adjacencies along PVCs.

Point-to-multipoint assumes broadcast capability by default; nonbroadcast can be specified, and neighbors must then be defined manually.


Broadcast Network


Point-to-point on Subinterfaces

Router(config)# interface serial0

Router(config-if)# no ip address

Router(config-if)# encapsulation frame-relay

Router(config)# interface serial0.1 point-to-point

Router(config-if)# ip address

Router(config-if)# frame-relay interface-dlci 51

Router(config)# interface serial0.2 point-to-point

Router(config-if)# ip address

Router(config-if)# frame-relay interface-dlci 52

Router(config)# router ip ospf 1

Router(config-router)# network area 0

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