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How to setup a VPN IPSEC between Zyxell USG200 and ForeFrontTMG

Trying to configure a VPN IPSEC between a Zyxell USG200 and Microsoft ForeFrontTMG

We found out that some poeple did it yet between Zywall 35 and Microsft ISA 2004
See: http://www.imamu.edu.sa/DContent/Articles/isa/IPSec_VPN_between_ISA_Server_2004_and_ZyWALL.pdf

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Software Engineer
Distinguished Expert 2019
Zywall 35 (ZyNOS, cpu unknown to me) is quite different from USG Series (Linux, PPC).
They have different processors & different OS's.

Anyway IPSEC still is IPSEC. You need the addresses of both gateways, a common secret key (pre shared key).
They both need the same parameters [ encryption etc. ] only the reference to local/remote is reversed between parties.
Your document uses DES, and that encryption has been cracked ages ago (56 bit keys are definitely too short).
3DES is usable, AES is most probably faster. [ anything above 128 suffices ].

Just collect the info you need on both boxes. Please come back and ask if you have more specific questions.