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CISSP study material · question 279 of 500

Two hosts need bidirectional IPsec-protected traffic with both AH and ESP applied. How many security associations are involved, and why?

  1. Three, because one association can be shared for the return path.
  2. One, because an association covers both directions and both protocols.
  3. Two, because one association per direction covers both protocols.
  4. Four, because an association is one-way and carries either AH or ESP but not both.
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Answer: D. Four, because an association is one-way and carries either AH or ESP but not both.

An association is a simplex connection that uses AH or ESP but never both, so each direction needs one association per protocol - two protocols in two directions gives four.

Source: RFC 4301 (IETF) — RFC 4301 > 4.1 Definition and Scope

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