OSPF Communication on OSPF Networks

The way in which OSPF routers address OSPF packets varies with the OSPF network type.

Broadcast Networks   For broadcast networks, OSPF routers use the following two reserved IP multicast addresses:

• 224.0.0.5 - AllSPFRouters: Used to send OSPF messages to all OSPF routers on the same network. The AllSPFRouters address is used for Hello packets. The DR and BDR use this address to send Link State Update and Link State Acknowledgment packets.
• 224.0.0.6 - AllDRouters: Used to send OSPF messages to all OSPF DRs (the DR and the BDR) on the same network. All OSPF routers except the DR use this address when sending Link State Update and Link State Acknowledgment packets to the DR.

Point-to-Point Networks   Point-to-Point networks use the AllSPFRouters address (224.0.0.5) for all OSPF messages.

NBMA Networks   NBMA networks have no multicasting capability. Therefore, the destination IP address of any Hello or Link State packets is the unicast IP address of a specific neighbor. The neighbor IP address is a required part of OSPF configuration for NBMA network links.

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