NAT Overload on the same TCP and UDP port

spinellicisco · ‎05-19-2011

Hi,

following my LAB scenario:

- my router is configured with NAT Overload for outgoing traffic;

- one LAN client generate two session, one TCP and one UDP using the port 5000 for both protocols, to an Internet client port 63000;

- during the translation the port of the TCP session remain that used by the LAN client, the port 5000.

- during the translation the port of the UDP session does not remain that used by the LAN client, the port 5000, but is used the port 1024.

I wanted know if is normal that the Cisco router change the port of the UDP session generated by the LAN client, using the port 1024 instead of the port 5000, since the two session use two different protocols, TCP and UDP.

Thanks.

Fabio

paolo bevilacqua · ‎05-19-2011

It is normal.

IOS NAT/PAT often remaps UDP ports even without apparent necessity, for reasons we could only second guess.