Site-to-site IPsec VPN — explain Phase 1 and Phase 2.

Phase 1 (IKE/ISAKMP) — establishes a secure channel for negotiating Phase 2. Negotiates: encryption algo (AES, 3DES), authentication (PSK or certs), DH group, lifetime. Two modes: Main Mode (6 messages, more secure) or Aggressive Mode (3 messages, faster but exposes identity). Phase 2 (IPsec) — establishes the actual data tunnel. Negotiates: transform set (ESP/AH + encryption + hash), proxy IDs (interesting traffic), tunnel mode (typical) or transport mode. Phase 1 SA is bidirectional; Phase 2 SAs are unidirectional pairs.

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