Hello Protocol Operational Variations

In broadcast networks (for example, Ethernet or Token Ring), each router advertises itself by periodically sending out multicast Hello packets, which allow neighbors to be discovered dynamically.

In NBMA networks (for example, frame relay, X.25, or ATM), the OSPF router can require some additional configuration information in order for the Hello protocol to operate correctly. This configuration is the protocol going out onto the network to find or elect the designated router, as previously discussed in the section "OSPF Network Types" in Chapter 2. In this case, the OSPF neighbor information must be manually configured, thereby allowing OSPF to send the Hello packets via unicast to the neighbor.

In point-to-point or point-to-multipoint networks (that is, Frame Relay topologies), the OSPF router sends out Hello multicast packets to every neighbor with which it can commu-

nicate directly. However, in point-to-multipoint networks, Hello packets can be sent as multicast if the data link layer replicated the packet. Neighbor information can be configured manually to indicate who to send a unicast Hello packet to if the data link replication does not work, such as in the ATM ARP server model.

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