Which IS-IS PDU is used to request a retransmission of a missing link-state packet?
A.
the complete sequence number PDU
B.
the partial sequence number PDU
C.
a hello PDU with a request PDU TLV specifying the missing LSP
D.
a hello PDU with a refresh PDU TLV requesting all LSPs
Partial sequence number PDUs (PSNPs)—Sent multicast by a receiver when it detects that it is missing a link-state PDU (when its link-state PDU database is out of date). The receiver sends a PSNP to the system that transmitted the CSNP, effectively requesting that the missing link-state PDU be transmitted. That routing device, in turn, forwards the missing link-state PDU to the requesting routing device.
A PSNP is used by an IS-IS router to request link-state PDU information from a neighboring router. A PSNP can also explicitly acknowledge the receipt of a link-state PDU on a point-to-point link. On a broadcast link, a CSNP is used as implicit knowledge. Like hello PDUs and CSNPs, the PSNP also has two types: Level 1 and Level 2.