3Com S7906E Configuration Manual page 722

SNP format
A sequence number PDU (SNP) acknowledges the latest received LSPs. It is similar to an
Acknowledge packet, but more efficient.
SNP involves Complete SNP (CSNP) and Partial SNP (PSNP), which are further divided into Level-1
CSNP, Level-2 CSNP, Level-1 PSNP and Level-2 PSNP.
CSNP covers the summary of all LSPs in the LSDB to synchronize the LSDB between neighboring
routers. On broadcast networks, CSNP is sent by the DIS periodically (10s by default). On point-to-point
networks, CSNP is only sent during the first adjacency establishment.
The CSNP packet format is shown in Figure 1-11.
Figure 1-11 L1/L2 CSNP format
PSNP only contains the sequence numbers of one or multiple latest received LSPs. It can acknowledge
multiple LSPs at one time. When LSDBs are not synchronized, a PSNP is used to request new LSPs
from neighbors.
Figure 1-12 shows the PSNP packet format.
1-11