IKEv1 negotiation phase 1 supports two negotiation modes. How many bidirectional exchanges does the main mode include?

Question

Options

• A1
• B3
• C2
• D4

• A
  11% (3)
• B
  81% (22)
• C
  4% (1)
• D
  4% (1)

Explanation

IKEv1 Phase 1 Main Mode consists of exactly 3 bidirectional exchanges (6 total messages), making B correct. The three exchanges handle SA parameter negotiation, Diffie-Hellman key exchange with nonces, and finally encrypted identity/authentication - each requiring a round-trip, yielding 3 pairs.

• A (1) is wrong because a single exchange cannot accomplish SA negotiation, key material sharing, and authentication separately - Main Mode deliberately separates these for security.
• C (2) is wrong because stopping at two exchanges would complete the DH exchange but leave authentication undone, which is required before Phase 1 concludes.
• D (4) is wrong; that exceeds Main Mode's design. Aggressive Mode, by contrast, collapses to fewer messages (3 total, not bidirectional pairs), trading security for speed.

Memory tip: Think "3 dates to commit" - SA proposals, key material, then authentication - each step needs a reply, so 3 bidirectional exchanges = 6 messages total in Main Mode.

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