> Source: [sk181157](https://support.checkpoint.com/results/sk/sk181157)

# sk181157 - VPN Gateway sends encrypted traffic through ESP but VPN peer sends encrypted traffic through NAT-T 

| Property | Value |
|----------|-------|
| Solution ID | sk181157 |
| Date Created | 2023-06-22 |
| Last Modified | 2023-06-27 |
| Technical Level | Advanced |
| Products | Security Gateway |
| Versions | R81.20, R81.10 (EOS), R81 (EOS) |

## Symptoms

- * A VPN Gateway sends encrypted traffic to the VPN peer gateway through ESP, but the peer gateway sends traffic through NAT-T.
* On the VPN Gateway, the outgoing interface IP address is different from the source IP address in the encrypted packets. The source IP address is on a different interface.
* The VPN Gateway is not behind a NAT device.

## Cause

In some configurations, the VPN Gateway uses an IP address that is not the outgoing interface IP address.  

The source IP address is usually configured on a loopback or dummy/unused interface. If the VPN peer uses NAT discovery (NAT-D), it thinks that the VPN Gateway is behind a NAT device. This is because the *send_single_nat_d_source* parameter is enabled on the VPN Gateway.The default configuration calculates the NAT-D hash on the outgoing interface IP, not the configured source IP.

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## Solution

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