What is LSAck?

An LSAck (Link State Acknowledgement) is the OSPF packet confirming receipt of link-state advertisements, making OSPF flooding reliable.

OSPF runs directly over IP as protocol 89, not over TCP, so it has to handle reliability itself. It uses five packet types: Hello (1), Database Description (2), Link State Request (3), Link State Update (4) and Link State Acknowledgement (5). When a router receives a Link State Update carrying one or more LSAs, it replies with an LSAck listing the headers of the LSAs it got, not the full LSAs. The header (type, link-state ID, advertising router, sequence number, checksum and age) is enough to identify the exact version being acknowledged. The sender keeps each LSA on a retransmission list for that neighbour and resends it every retransmit interval (5 seconds by default) until it is acknowledged.

Acknowledgements can be delayed briefly so several can be bundled into one packet, and a router can acknowledge implicitly by flooding the same LSA back to the neighbour that sent it. On a broadcast segment, acknowledgement follows the DR model: DROthers acknowledge to the DR and BDR, and the DR acknowledges to everyone. You rarely configure anything about LSAcks, but they show up when troubleshooting: if a neighbour keeps retransmitting the same LSAs, acknowledgements are being lost, often because of a unidirectional link, an MTU problem or a filter dropping OSPF multicast. In a packet capture, seeing LSUs answered by LSAcks is the sign that flooding is working. The OSPF Fundamentals lesson walks through the packet types and how flooding builds the shared database.