What is LSU?

An LSU (Link State Update) is the OSPF packet that carries complete LSAs between neighbours, used both to answer requests and to flood changes through an area.

OSPF has five packet types: Hello (1), Database Description (2), Link State Request (3), Link State Update (4) and Link State Acknowledgement (5). The LSU is the only one that carries full LSAs, the records that describe each router's links and the networks it knows about. A single LSU holds a count followed by one or more LSAs. It's used in two situations:

• Building an adjacency. In the Loading state a router sends Link State Requests for LSAs it's missing or has older copies of, and the neighbour replies with LSUs containing them.
• Flooding. When something changes, such as a link going down or a cost being edited, the originating router issues a new LSA with a higher sequence number and floods it in LSUs. Each router that receives a newer copy installs it in its LSDB and floods it out of its other interfaces.

OSPF runs directly over IP, not TCP, so flooding is made reliable by the protocol itself. Every LSA received in an LSU must be acknowledged, normally with an LSAck; a router keeps retransmitting the LSA to a neighbour until it's acknowledged, every 5 seconds by default. On broadcast networks LSUs are multicast: routers other than the DR and BDR send them to 224.0.0.6, and the DR refloods to everyone on 224.0.0.5. When adjacencies sit in Loading, or the same LSA keeps being retransmitted, look at MTU mismatches or packet loss on the link. The OSPF Fundamentals lesson shows how adjacencies form and how LSAs are flooded and used for SPF.