What is LSR?

An LSR (Link State Request) is an OSPF packet that a router sends to a neighbour to ask for specific LSAs it is missing or holds an older copy of.

OSPF uses five packet types: Hello (1), Database Description or DBD (2), Link State Request (3), Link State Update or LSU (4) and Link State Acknowledgement or LSAck (5). When two routers form an adjacency, they first swap DBD packets in the Exchange state. These list the headers of every LSA each router holds, not the full contents. Each router compares the list with its own LSDB and notes any LSA it lacks or that is newer on the other side. It then asks for exactly those, identifying each one by three fields: the LS type, the Link State ID and the Advertising Router. The neighbour answers with LSU packets containing the full LSAs, and requests that go unanswered are sent again after the retransmit interval. Once nothing is left to ask for, the neighbour moves from Loading to Full.

This request-only-what-you-need step is why OSPF adjacencies form quickly even with a large database: routers that already agree on most of the LSDB transfer very little. You rarely look at LSRs directly unless you capture an adjacency forming with a packet capture, but they explain the Loading state. A neighbour that stays stuck in Loading usually means requested LSAs aren't arriving, for example because large LSU packets are being dropped by an MTU problem or a filter. LSRs are also only used during database synchronisation; after the adjacency is Full, changes are flooded with LSUs and acknowledged with LSAcks, with no request needed. The OSPF Fundamentals lesson walks through every neighbour state, including Exchange and Loading.