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In FortiOS v7.0.1 and above, each IPsec tunnel is identified by the tunnel ID. The tunnel ID is automatically assigned with the remote gateway IP address in phase 1 configuration.
For example :
show vpn ipsec phase1-interface
config vpn ipsec phase1-interface
edit "ToLotus"
set interface "port1"
set peertype any
set net-device disable
set proposal aes128-sha256 aes256-sha256 aes128-sha1 aes256-sha1
set comments "VPN: ToLotus (Created by VPN wizard)"
set wizard-type static-fortigate
set remote-gw 10.47.2.140
diagnose vpn ike gateway list
vd: root/0
name: ToLotus
version: 1
interface: port1 3
addr: 10.47.1.13:500 -> 10.47.2.140:500
tun_id: 10.47.2.140/::10.47.2.140
remote_location: 0.0.0.0
network-id: 0
created: 298s ago
IKE SA: created 2/2 established 2/2 time 10/4510/9010 ms
IPsec SA: created 1/2 established 1/1 time 20/20/20 ms
If there is any change in the configuration of the remote gateway in phase1, the tunnel ID will remain as first configured.
For example, the gateway of the same tunnel is now changed:
show vpn ipsec phase1-interface
config vpn ipsec phase1-interface
edit "ToLotus"
set interface "port1"
set peertype any
set net-device disable
set proposal aes128-sha256 aes256-sha256 aes128-sha1 aes256-sha1
set comments "VPN: ToLotus (Created by VPN wizard)"
set wizard-type static-fortigate
set remote-gw 10.47.18.140
diagnose vpn ike gateway list
vd: root/0
name: ToLotus
version: 1
interface: port1 3
addr: 10.47.1.13:500 -> 10.47.18.140:500
tun_id: 10.47.2.140/::10.47.2.140
remote_location: 0.0.0.0
network-id: 0
created: 64s ago
IKE SA: created 1/1 established 1/1 time 0/0/0 ms
IPsec SA: created 1/1 established 1/1 time 110/110/110 ms
Note that tun_id remained as 10.47.2.140. Flushing the tunnel will not make any difference. However, the tunnel should be working as intended, as the tunnel ID is only used as an identifier.
If there is a need to change the tun_id to reflect the new gateway IP, choose one of the following steps:
- Reconfigure it, creating a new tunnel.
- Reboot the FortiGate.
Note: The remote-gateway IP selected as tunnel-id is only for the normal site-to-site VPN scenario. In the ADVPN hub-and-spoke configuration, the tunnel-id on the hub will be the IPsec tunnel interface IP of each spoke instead of remote-gateway IP once the 'auto-discovery-receiver' is enabled on the spoke.
This is to ensure the hub will be able to forward traffic to the correct sub-tunnel in the BGP route. If this option is disabled, the remote-gateway IP will become the tunnel ID of each sub-tunnel on the hub, which will be different from the next-hop IP in the BGP route. In that case, the hub will get confused, and the hub-and-spoke topology will stop working.
Related documents:
Technical Tip: IPsec VPN route shows a different gateway IP than a remote IP address
Dedicated tunnel ID for IPsec tunnels
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