LSPs are flooded through the network by sequence number PDUs (SNPs). The number sequencing ensures that the LSP flooding is performed in a reliable fashion by each IS-IS router. As mentioned at the beginning of this chapter, SNPs come in two types:
• CSNPs— Include all LSPs in the link state database.
• PSNPs— Include some LSPs in the link state database.
On point-to-point IS-IS networks, PSNPs are used to acknowledge CSNPs. The default configuration on Cisco routers for LSP transmission is 5 seconds; however, this interval can be changed manually. The range for this value is from 0 to 65,535.
On broadcast IS-IS networks, routers receive multicast LSPs from each neighbor. As opposed to having each router on the network acknowledge each received LSP, the DIS sends CSNPs in response, containing its link state database. CSNPs are multicasted every 10 seconds by default; however, this interval can be changed manually. The range for this value is from 0 to 65,535.
L1 CSNPs are sent to the multicast address AllL1ISs; L2 CSNPs are sent to AllL2ISs.
Upon receipt of the CSNP, each router in the broadcast network compares the CSNP against its link state database. If a router is missing an LSP that is contained within the CSNP, that same router multicasts a PSNP containing the missing LSP. The DIS responds with the requested LSPs to any PSNPs.
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