En fait, je pense que le problème est multiples :
1°) - Nortel Contivity ne fait pas parti des VPN supportés par vpnc.
2°) - Il y a bien une version vpnc-nortel mais qui me semble aussi nécessiter le group ID.
1°)
http://www.unix-ag.uni-kl.de/~massar/vpnc/ :
vpnc - client for cisco vpn concentrator
vpnc is supposed to work with:
Cisco VPN concentrator 3000 Series
Cisco IOS routers
Cisco PIX / ASA Zecurity Appliances
Juniper/Netscreen
2°)
http://svn.unix-ag.uni-kl.de/vpnc/branches/vpnc-nortel/README
A VPN client compatible with Cisco's EasyVPN equipment.
Supports IPSec (ESP) with Mode Configuration and Xauth.
Supports only
shared-secret IPSec authentication with Xauth,
AES (256, 192, 128), 3DES, 1DES, MD5, SHA1,
DH1/2/5 and IP tunneling.
...
If you don't know the Group ID and Secret string, ask your
administrator.
...