A PDU is transmitted to the neighbor by an IS after an ISH is received from the neighbor. The purpose of this is to determine whether the neighbor is a level 1 or a level 2 intermediate system.
After the neighbor is determined, the router then sends the CSNP on the point-to-point link. CSNPs are sent only the first time for the synchronization of a database. If the neighbor router discovers that it needs a newer instance of the LSP, it can request the LSP via the PSNP. The PSNP is also used for the acknowledgment of the LSP.
A router considers an LSP acknowledged when a PSNP is received with the same sequence number. If the remote router has an older version, it sends a PSNP with the sequence number. The local router notices that the remote router is missing the newer copy of the LSP, so it floods the newer LSP to the neighbor and sets the SRM.
Upon receiving the newer copy, the remote router installs the newer copy in its database and floods it further. It then sends the PSNP back to the local router, indicating the receipt of the LSP. Upon acknowledgment, the local router clears the SRM bit. In the case of point-to-point links, the SRM bit is cleared only after a PSNP is received indicating acknowledgment. Figure 10-5 shows the flooding process over point-to-point links.
Figure 10-5. Flooding Over Point-to-Point Links
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