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EBGP exchanges internet (i.e. global) prefixes/routes from other Autonomous systems (EBGP peers). To reach those prefixes, someone should distribute/advertise across the internal network, IBGP comes to the rescue. IBGP advertises these global prefixes to the routers within the Autonomous System (AS). Why not IGP does the same? As IGP is quite chatty (i.e. in ...


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"Flooding" means LSA is sent to all routers in the area (not just the given network), whereas multicasting means the OSPF pkt (with LSA in it) is listened to by all OSPF routers on the given network (not the the whole area). Unicasting would require more packets, so it would be less efficient. (224.0.0.9 is for RIPv2 BTW, not OSPF; "all OSPF ...


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You can use OSPF in P2P mode only on P2P links - e.g. a serial link with PPP on top, or with most kinds of tunnels. On broadcast networks like Ethernet OSPF needs to use multicast for neighbor discovery - even if two routers are directly connected together since Ethernet requires addressing, it is a broadcast network after all. On some systems you can skip ...


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Does OSPF have to work in DR/BDR mode on Ethernet? No. By default, Ethernet interfaces are broadcast type, and they elect DR/BDR. But you can change it with ip ospf network point-to-point command that eliminates the DR/BDR election. Is it rather default configuration of cisco routers that is supposed to be overwritten. Is it bad configuration to still ...


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Cisco has the ip ospf network point-to-point interface command for such situations. You could also use the neighbor statement under the OSPF router configuration to use unicast. Having a DR/BDR on an ethernet point-to-point link really is not a problem just because the traffic gets sent via multicast. On such a link, unicast or multicast achieves the same ...


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