partial sequence numbers PDU (IS) (PSNP)
A Partial Sequence Number PDU (PSNP) is a Protocol Data Unit (PDU) that requests Link State Packets( LSP) from an Intermediate System( IS). This protocol data unit is used to make requests to the neighboring router for routing information.
The PSNP is one of four message types recognized by the Intermediate System to Intermediate System Protocol( IS-IS).