OSPF - Link State Updates (LSUs)

A Link State Update (LSU) packet in OSPF is one of several types of OSPF protocol packets. The LSU packet plays a crucial role in the OSPF operation by carrying link-state advertisements (LSAs) to other routers within the same OSPF area or the entire routing domain, depending on the configuration and OSPF area types.

According to the RFC:

Link State Update packets are multicast on those physical networks that support multicast/broadcast. In order to make the flooding procedure reliable, flooded LSAs are acknowledged in Link State Acknowledgment packets. If retransmission of certain LSAs is necessary, the retransmitted LSAs are always sent directly to the neighbor.

So LSUs are always sent as multicast, unless they are responding to a specific LSR, in which case they use unicast.





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