IS-IS Packets

IS-IS uses the following protocol data units (PDUs) to exchange protocol information:

IS-IS hello (IIH) PDUs—Broadcast to discover the identity of neighboring IS-IS systems and to determine whether the neighbors are Level 1 or Level 2 intermediate systems.
Link-state PDUs (LSPs)—Contain information about the state of adjacencies to neighboring IS-IS systems. LSPs are flooded periodically throughout an area.
Complete sequence number PDUs (CSNPs)—Contain a complete list of all LSPs in the IS-IS database. CSNPs are sent periodically on all links, and the receiving systems use the information in the CSNP to update and synchronize their LSP databases. The designated router multicasts CSNPs on broadcast links in place of sending explicit acknowledgments for each LSP.
Partial sequence number PDUs (PSNPs)—Multicast by a receiver when it detects that it is missing an LSP; that is, when its LSP database is out of date. The receiver sends a PSNP to the system that transmitted the CSNP, effectively requesting that the missing LSP be transmitted. That router, in turn, forwards the missing LSP to the requesting router.