Problem with IPSec VPN tunnel to remote site

Hi experts,

We need to setup an IPSec VPN tunnel to a remote site. Other remote site hardware is unkown, but we do know the IPSec settings. Phase 1 and Phase 2 have been configured and firewall policies are defined.

In our Fortigate logs we get this during a setup of the tunnel:

error       dpd                   IPsec connection failure on the tunnel to <remote ip>:500       dpd_failure       
notice      negotiate             Initiator: tunnel <remote ip>, transform=ESP_AES, HMAC_SHA1       success
notice       negotiate             Initiator: sent <remote ip> quick mode message #2 (DONE)       success
notice       install_sa             Initiator: tunnel <local ip>/<remote ip> install ipsec sa       
notice       negotiate             Initiator: sent <remote ip> quick mode message #1 (OK)             success       
notice       negotiate             Initiator: parsed <remote ip> main mode message #3 (DONE)       success
notice       negotiate             Initiator: sent <remote ip> main mode message #3 (OK)             success
notice       negotiate             Initiator: sent <remote ip> main mode message #2 (OK)             success
notice       negotiate             Initiator: sent <remote ip> main mode message #1 (OK)             success
notice       delete_phase1_sa       Deleted an Isakmp SA on the tunnel to <remote ip>:500

The dpd_failure message has id 23011. According to fortigate this means:
1.11. Message ID: 23011
Message:       loc_ip=<local_ipaddress> loc_port=<local_port> rem_ip=<> rem_port=<> out_if=<> vpn_tunnel=<ip_address> cookies=<> action=dpd status=dpd_failure msg="IPSec connection failure"
Meaning:       IPSec connection failure.

(see url

The problem is here we need a deeper analysis of what exactly is going wrong.
dpd_failure = dead peer detected failure, but since it's replying on the first phases, the Fortigate can reach the other site.

Anyone a suggestion?

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Although I am not too familiar with Fortigate, I think the remote end is behind a NAT device or is not responding with HELLO or ACK to your device, as a result your device thinks that the remote end is dead and reinitiates the SA.

Looking at the logs, Phase I and Phase II completes; after this, there is no transmit of traffic over the VPN tunnel and your device checks to see of the remote peer is alive (dpd); it send packets does not get any HELLO or ACK and thinks that the remote peer is actually dead, bringing down the negotiated SA.

I would suggest you to get some details about the remote device. Also, if possible to deactivate dpd on fortigate, you might re-enable dpd later.

Thank you.

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techneitsolutionsAuthor Commented:
Must be something between the fortigate and the remote device, since i've tried settings up a second tunnel for testing purpose. With the same settings between two fortigate devices. This worked from the moment i activated the tunnel.

So i'll try your advice and disabled the dpd check.

I know the remote device is not a Fortigate, but i'll see i can get some more information soon.
techneitsolutionsAuthor Commented:
Okay this did solve the problem.

I also found someone with the same problem between a Fortigate and a Cisco. He also had to disable dtd on the Fortigate so that the VPN tunnel would become operational.

Thank you for the info it would help everyone! :)
I am going for a wild guess but did you defined a firewall rule to allow some or all protocols, depending of your needs  (with rule ENCRYPT) the traffic flowing from site A to site B once the connection has been established ?

Clearly the dead peer detection could be a cause of lost syncs, however if you don't setup a firewall rule the connexion isn't going to be maintained just after phase I and II.
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