As shown in the figure, the IS-IS neighbor relationship between R1 and R2 has just been established, and the database synchronization process between R1 and R2 is correct?

Question

Options

• AThe packet sent by R1 to R2 is a hello packet.
• BThe packet sent by R1 to R2 is an LSP packet.
• CThe packet sent by R1 to R2 is a PSNP packet.
• DThe packet sent by R1 to R2 is a CSNP packet.

• A
  10% (3)
• B
  3% (1)
• C
  3% (1)
• D
  83% (25)

Explanation

Option D is correct because after an IS-IS adjacency is first established, the database synchronization process begins with the exchange of CSNP (Complete Sequence Number PDU) packets. R1 sends a CSNP to R2 to advertise a complete summary of its Link State Database (LSDB), allowing R2 to compare and identify any missing or outdated LSPs.

Why the distractors are wrong:

• A (Hello/IIH): Hello packets are used to discover and maintain neighbors - that phase is already complete by the time this question begins.
• B (LSP): LSPs carry actual topology data and are exchanged after the CSNP comparison reveals what's missing, not as the opening move of synchronization.
• C (PSNP): PSNPs are sent by the receiving router in response to a CSNP, either to acknowledge received LSPs or to request specific missing ones - they come after the CSNP, not before.

Memory tip: Think of the sync flow as "Show → Ask → Receive": R1 shows its full DB summary (CSNP), R2 asks for what it's missing (PSNP), then R1 delivers the actual data (LSP). The question asks about the very first packet, which is always the "Show" - the Complete Summary = CSNP.

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