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Chapter 24 IPSec VPN
Main mode takes six steps to establish an IKE SA.
Steps 1 - 2: The ISG50 sends its proposals to the remote IPSec router. The remote IPSec router
selects an acceptable proposal and sends it back to the ISG50.
Steps 3 - 4: The ISG50 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 ISG50 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 ISG50 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 256 VPN/NAT Example
ISG
A
Y
X
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 feature, router X and
router Y can establish a VPN tunnel as long as the active protocol is ESP. (See
Active Protocol on
page 392
for more information about active protocols.)
If router A does not have an IPSec pass-thru or if the active protocol is AH, you can solve this
problem by enabling NAT traversal. In NAT traversal, router X and router Y add an extra header to
the IKE SA and IPSec SA packets. If you configure router A to forward these packets unchanged,
router X and router Y can establish a VPN tunnel.
You have to do the following things to set up NAT traversal.
• Enable NAT traversal on the ISG50 and remote IPSec router.
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ISG50 User's Guide
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Troubleshooting
