Configuration on the Linksys: (obvious configuration parameter like local subnet and remote ip are left away)

IPSEC Setup:

Keying Mode: IKE with Preshared key
Phase1 DH Group: Group2
Phase1 Encryption: 3DES
Phase1 Authentication: SHA1
Phase1 SA Life Time: 28800
Perfect Forward Secrecy: unchecked

Phase2 Encryption: 3DES
Phase2 Authentication: SHA1
Phase2 SA Life Time: 28800
Pre-Shared Key: xxxxxx

Advanced:

Aggressive Mode: unchecked
Compress: unchecked
Keep Alive: unchecked
AH Hash Algorithm unchecked
NetBIOS Broadcast: unchecked
Nat Traversal: checked
Dead Peer Detection: 30 Seconds


Configuration on Endian Firewall (Advanced Configuration):

Compression: unchecked
IKE Encryption: 3DES
IKE Integrity: SHA1
IKE Lifetime: 8 hours
IKE Group Type: MODP-1024
ESP Encryption: 3DES
ESP Integrity: SHA1
ESP Keylife: 8 Hours
ESP Group Type: Phase1 Group
Use Only Proposed Settings: checked


Thanks goes to Jesse Shumaker and Thomas Heimann