Chapter 12 IPSec VPN
P-2812HNU-51c User’s Guide
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With the use of AH as the security protocol, protection is extended forward into
the IP header to verify the integrity of the entire packet by use of portions of the
original IP header in the hashing process.
Tunnel Mode
Tunnel mode encapsulates the entire IP packet to transmit it securely. A Tunnel
mode is required for gateway services to provide access to internal systems.
Tunnel mode is fundamentally an IP tunnel with authentication and encryption.
This is the most common mode of operation. Tunnel mode is required for
gateway to gateway and host to gateway communications. Tunnel mode
communications have two sets of IP headers:
• Outside header: The outside IP header contains the destination IP address of
the VPN gateway.
• Inside header: The inside IP header contains the destination IP address of the
final system behind the VPN gateway. The security protocol appears after the
outer IP header and before the inside IP header.
12.5.3 IKE Phases
There are two phases to every IKE (Internet Key Exchange) negotiation – phase 1
(Authentication) and phase 2 (Key Exchange). A phase 1 exchange establishes an
IKE SA and the second one uses that SA to negotiate SAs for IPSec.
Figure 136 Two Phases to Set Up the IPSec SA
In phase 1 you must:
• Choose a negotiation mode.
• Authenticate the connection by entering a pre-shared key.
• Choose an encryption algorithm.
• Choose an authentication algorithm.
