So, you've tried many combinations in an attempt to configure your Mac OS X L2TP/IPsec VPN clients to use IKE authentication with certificates with your ISA Server 2006, certificates issued for the Mac VPN Clients and ISA by an internal Windows Enterprise CA.
You may have a couple of certificates installed on ISA issued by this CA, so you have troubles identifying which certificate is ISA picking for IKE authentication, if this certificate has the "correct" fields required by the Mac OS X L2TP/IPsec VPN clients' security checks.
Till now this "game" left you fuming.
I've mentioned in the first part of this article about the posibility of using a separate CA to issue certificates just for the Mac OS X L2TP/IPsec VPN clients and ISA, certificates that can be used for IKE authentication.
If you have not gave up, and you are still searching for a solution, one such solution can be to use an OpenSSL CA to issue the needed certificates for IKE authentication.
Your Windows L2TP/IPsec VPN clients will continue to use the same certificates they used before for IKE authentication.
We will use this OpenSSL CA just to issue to ISA a "valid"(from the perspective of the Mac OS X L2TP/IPsec VPN clients) certificate and also to issue certificates for the Mac OS X L2TP/IPsec VPN clients.
I'm using an OpenSSL CA because it can easily issue the "correct" certificate for ISA that will work with Mac OS X L2TP/IPsec VPN clients.
We have seen that it is recommended to have installed on ISA a certificate with no EKU field and with the SAN field containing a DNS Name, a FQDN which the Mac OS X L2TP/IPsec VPN clients will use as their VPN server address.
Since only one certificate will be installed on ISA issued by this CA, you will know for sure what certificate will be used for the IKE authentications between ISA and the Mac OS X L2TP/IPsec VPN clients. No more "games".