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Why are OSPF neighbors manually configured on NBMA networks even when you enabled it with the broadcast keyword in frame map statements?

Example is NBMA point-to-multipoint access.

Thanks.

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2 Answers 2

Frame relay does not support real broadcast, it uses pseudo broadcast, meaning that it replicates incoming multicast/broadcast to all DLCIs that have the broadcast keyword on the frame-relay map statements. DLCIs learned via inverse ARP will always have broadcast enabled.

When you choose the network type you tell OSPF how to behave, is DR/BDR used and is multicast used?

With non-broadcast or point-to-multipoint non-broadcast (Cisco specific) you need to define neighbor statements. The reason being that you have told OSPF that you have no broadcast capability so you want to use unicast for the control plane traffic. OSPF therefore expects that you configure neighbor statements before it starts sending hellos.

I've had a few facepalm moments in the past studying for the CCIE where I did enable non-broadcast and no adjacencies were coming up. Debugs showed no hellos going out which is usually a sign you forgot a neighbor statement or are running passive-interface.

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can it work on point-to-multipoint frame-relay interface same like ethernet segment? –  Jim Apr 5 at 17:36
    
Not sure what you are asking but P2MP can be used on both Frame relay and Ethernet segments. It's mostly used to overcome NBMA limitations though. It could be useful in Private VLAN scenarios. –  Daniel Dib Apr 6 at 4:22

This network type is used on networks that have no broadcast/multicast capability, such as frame-relay, ATM, SMDS, & X.25. The key point is that these layer 2 protocols are unable to send broadcasts/multicasts.

The key thing to remember is that the neighbor command is restricted to the DR & BDR only . The neighbor command allows the router to send hello’s to those specific neighbors. Those messages are sent as unicast messages. The reason the neighbor command is only required on one side between neighbors, is because hello’s are sent if hellos are received. Hello’s are sent every 30 seconds by default, on this network type, or upon receiving an hello packet. So therefore, hello’s will be able to be exchanged between the two neighbors.

Quick Facts

  • All OSPF packets are sent unicast to each neighbor

  • Default timers are Hello 30 secs, Dead 120 secs

  • DR/BDR election occurs.

  • DR/BDR must be connected to all other routers

  • Neighbor statements must be manually configured on the DR/BDR routers

  • Typical in a full mesh network, although it can be used in a hub and spoke design

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