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I am trying to replace a Cisco router with a GNU Debian/Linux router. This Cisco router terminates a VPN connection that was setup using VTI. Could someone please help me what software and configuration to use on Debian?

This is a summary from the working configuration on the Cisco router. I have replaced some sensible parts with text in angle brackets. I have no control over the other end of the IPSEC tunnel:

crypto pki trustpoint TP-self-signed-<keyid>
 enrollment selfsigned
 subject-name cn=IOS-Self-Signed-Certificate-<keyid>
 revocation-check none
 rsakeypair TP-self-signed-<keyid>
no ip subnet-zero
no ip source-route
ip auth-proxy max-nodata-conns 3
ip admission source-interface Vlan1
ip admission max-nodata-conns 3
crypto isakmp policy 1
 encr 3des
 authentication pre-share
 group 2
crypto isakmp key <password> address <remote public IP>
crypto ipsec transform-set <xform name> esp-3des esp-sha-hmac
crypto ipsec profile <profile name>
 set transform-set <xform name>
interface Tunnel98
 ip address <local private tunnel IP>
 no ip redirects
 ip mtu 1400
 ip tcp adjust-mss 1360
 tunnel source Dialer1
 tunnel destination <remote public IP>
 tunnel mode ipsec ipv4
 tunnel protection ipsec profile <profile name>
interface Vlan1
 ip address <local private IP>
 no ip redirects
 no ip unreachables
 no ip proxy-arp
 ip nat inside
 ip virtual-reassembly
interface Dialer1
 ip address negotiated
 ip access-group to-internet out
 ip mtu 1492
 ip nat outside
 ip virtual-reassembly
 encapsulation ppp
 dialer pool 1
 dialer-group 1
 no cdp enable
 ppp authentication pap callin
 ppp pap sent-username <ppp username> password 7 <ppp password>
 ppp ipcp dns request
 ppp ipcp route default
no ip forward-protocol nd
ip route Dialer1
ip route 10.<remote private subnet 1>.0.0 <remote private tunnel IP> name <remote net 1 description>
ip route 10.<remote private subnet 2>.0.0 <remote private tunnel IP> name <remote net 2 description>
ip nat inside source list natacl interface Dialer1 overload
ip access-list standard natacl
 permit <local private net>
ip access-list extended to-internet
 deny   ip any
 deny   ip any
 deny   ip any
 permit ip any any
dialer-list 1 protocol ip permit
no cdp run

Thanks for your insights in advance!

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VTI support has just been added to Linux. 3.6 Kernel

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To terminate an IPsec VPN you probably want to look at Openswan or strongSwan.

The Openswan wiki has an entry on interoperability with Cisco:

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