why does BIG IP change ports from one inbound interface to another outbound?

Hello friends,

 

I have just configured a NAT in the F5 in order to handle VPN IPsec Traffic. I got sure about enabling NAT-T in both ends, so the VPN IPsec should have worked fine. But, I did not succeed. I have captured some traffic in the interface which is in the front end and the interface which is in the backend.

 

FRONT END F5´s INTERFACE

 

 

BACK END F5´s INTERFACE

 

 

So, it is the same kind of traffic. Why does the F5 change the port from one interface to the other? I know it is just a source port, buy why 0?

 

Thanks in advance. Any comment will be welcome.

 

OL