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    CCNP BSCI Notes – OSPF Network Topologies

    By Raiy Wong | March 30, 2010

    OSPF Network Topology Options

    OSPF assumes a subnet is broadcast-capable by default.

    OSPF Network Types

    Broadcast multiaccess

    Point-to-point

    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.

    clip_image002

    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.

    clip_image004

    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:

    clip_image006

    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.

    clip_image008

    Broadcast Network

    clip_image010

    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 10.1.1.1 255.255.255.0

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

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

    Router(config-if)# ip address 10.1.2.1 255.255.255.0

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

    Router(config)# router ip ospf 1

    Router(config-router)# network 10.1.1.1 0.0.0.255 area 0

             

    Topics: CCNP, CCNP Notes | No Comments »

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