Routers running a dynamic routing protocol, such as OSPF, exchange messages between neighbors on the same subnet or link. Routers only need to send and receive routing protocol messages with their directly connected neighbors. These messages are always sent from the source IPv4 address of the router doing the forwarding.
IPv6 link-local addresses are ideal for this purpose. An IPv6 link-local address enables a device to communicate with other IPv6-enabled devices on the same link and only on that link (subnet). Packets with a source or destination link-local address cannot be routed beyond the link from where the packet originated.
As shown in the figure, OSPFv3 messages are sent using:
- Source IPv6 address - This is the IPv6 link-local address of the exit interface.
- Destination IPv6 address - OSPFv3 packets can be sent to a unicast address using the neighbor IPv6 link-local address. They can also be sent using a multicast address. The FF02::5 address is the all OSPF router address, while the FF02::6 is the DR/BDR multicast address.