In a broadcast network, IS-IS establishes neighbor relationships through (___) handshakes.

Question

Options

• A3
• B
• C
• D

• A
  84% (21)
• B
  4% (1)
• C
  4% (1)
• D
  8% (2)

Explanation

IS-IS on broadcast networks (such as Ethernet LAN segments) establishes neighbor adjacencies through a 3-way handshake. In the first step, Router A sends an IS-IS Hello (IIH) PDU; in the second, Router B receives it, adds Router A to its neighbor list, and sends back its own Hello that includes Router A's System ID; in the third, Router A receives B's Hello, sees its own System ID listed, and sends a confirming Hello listing Router B - at which point both sides have verified bidirectional reachability. Options B, C, and D are not provided in this question, so no specific distractors can be evaluated, though a common point of confusion is that IS-IS point-to-point links may use a 2-way adjacency model by default (extended to 3-way via RFC 3373), making it tempting to select 2 for broadcast networks as well. Note also that OSPF on broadcast networks goes through a more elaborate multi-state process (2-Way, ExStart, Exchange, Loading, Full), which is distinct from IS-IS's Hello-based handshake.

Memory tip: Picture three Hellos flying across the broadcast segment - send, reply with your name, confirm their name - three steps to be sure both sides truly see each other before committing to the adjacency.

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