This is the number of IS-IS PSNP frames seen in this direction at this level. Since PSNPs serve as both point-to-point LSP acknowledgments and explicit requests for missing LSPs after a CSNP reveals a gap, a spike in this counter is the signature of either heavy point-to-point acknowledgment traffic during a topology change or an active database resynchronization. For example, a burst in PSNP count on a LAN circuit right after a CSNP interval would indicate one or more neighbors on that segment discovered they were missing LSPs and are now requesting them.