The following PDUs are used by IS-IS to exchange protocol information:
IS-IS hello PDU
Routers with IS-IS enabled send hello PDUs to IS-IS-enabled interfaces to discover neighbors and establish adjacencies.
Link-state PDUs
Contain information about the state of adjacencies to neighboring IS-IS systems. LSPs are flooded periodically throughout an area.
Complete sequence number PDUs
In order for all routers to maintain the same information, CSNPs inform other routers that some LSPs can be outdated or missing from their database. CSNPs contain a complete list of all LSPs in the current IS-IS database.
Partial sequence number PDUs (PSNPs)
PSNPs are used to request missing LSPs and acknowledge that an LSP was received.