In OSPF, routers from down to full state,what's kind of networks(broadcast multicast unicast) used per state to exchange information? I always heard and these two address, and was told those two multicast address is the most difference with RIPV1(Because rip1 use broadcast), so does that mean in OSPF routers always use these two multicast address to exchange information? If that, But why still seen broadcast and unicast in OSPF? I got confused!

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Multicast address is used for neighbor discovery (Hello messages). Then database exchange is made on a peer to peer basis (with ip address of routers).

The kind of OSPF networks is mostly used to indicate how each neighbor will communicate with other neighbors. For example, in broadcast mode, A router is designated to be the the hub for database exchanges with other neighbors (DR). And a backup router (BDR) is also designated in case of failure of the DR.

  • OSPF supports lots of network types. Case1: In broadcast and NBMA network, routers will send or to find neighbors, after all routers are in full adj state, they will exchange information using unicast address, is this correct? Case2: in peer to peer network, how the routers find neighbors, using unicast address or multicast address( – sky May 4 '18 at 10:45
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    Multicast is used by default to discover neighbors, unless you manually configure a neighbor, in which case, unicast is used for hello process. NBMA networks don't support multicast, so you will have to manually define neighbors. Point2Point networks may support multicast (ie ipsec tunnel), so multicast can be used. Database Exchange is on a peer2peer basis, so unicast is used. – omuffat May 6 '18 at 11:27

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