ZyXEL ZyWALL USG 2000 User Guide - Page 474

Chapter 25 IPSec VPN Negotiation Mode There are two negotiation modes--main mode and aggressive mode. Main mode provides better security, while aggressive mode is faster. Main mode takes six steps to establish an IKE SA. Steps 1 - 2: The ZyWALL sends its proposals to the remote IPSec router. The remote IPSec router selects an acceptable proposal and sends it back to the ZyWALL. Steps 3 - 4: The ZyWALL and the remote IPSec router exchange pre-shared keys for authentication and participate in a Diffie-Hellman key exchange, based on the accepted DH key group, to establish a shared secret. Steps 5 - 6: Finally, the ZyWALL and the remote IPSec router generate an encryption key (from the shared secret), encrypt their identities, and exchange their encrypted identity information for authentication. In contrast, aggressive mode only takes three steps to establish an IKE SA. Aggressive mode does not provide as much security because the identity of the ZyWALL and the identity of the remote IPSec router are not encrypted. It is usually used in remote-access situations, where the address of the initiator is not known by the responder and both parties want to use pre-shared keys for authentication. For example, the remote IPSec router may be a telecommuter who does not have a static IP address. VPN, NAT, and NAT Traversal In the following example, there is another router (A) between router X and router Y. Figure 340 VPN/NAT Example X A Y If router A does NAT, it might change the IP addresses, port numbers, or both. If router X and router Y try to establish a VPN tunnel, the authentication fails because it depends on this information. The routers cannot establish a VPN tunnel. Most routers like router A now have an IPSec pass-thru feature. This feature helps router A recognize VPN packets and route them appropriately. If router A has this 474 ZyWALL USG 2000 User's Guide