On an IS-IS network, routers send LSPs to exchange link state information. LSPs are classified into Level-1 LSPs and Level-2 LSPs and have the same format. Which of the following parts constitute…

Question

Options

• ALSP Number
• BPseudonode ID
• CSystem ID
• D

• A
  73% (30)
• B
  7% (3)
• C
  17% (7)
• D
  2% (1)

Explanation

In IS-IS, the LSP ID is an 8-byte field made up of three components: the System ID (6 bytes), the Pseudonode ID (1 byte), and the LSP Number (1 byte) - meaning options A, B, and C are all correct parts of the LSP ID, not distractors. The System ID uniquely identifies the originating router, the Pseudonode ID distinguishes between a router's own LSP (value 0x00) and a pseudonode LSP generated on a broadcast link (non-zero value), and the LSP Number acts as a fragment index when the LSP is too large to fit in a single PDU and must be split. Option D does not correspond to any recognized field in the IS-IS LSP ID structure. The "Correct Answer: A" notation here appears to refer to this being a multiple-select question where all three named options are valid, not a single-choice scenario.

Memory tip: Remember the acronym SPN - System ID, Pseudonode ID, Number (fragment) - as the three-part recipe that makes every IS-IS LSP uniquely identifiable across the network.

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