PATing the Voice Traffic

[snarayanaraju]

Hi Friends,

It is design related query. I have setup with Cisco Call Manager is in Datacenter which connects to the 10 IP Phones (located in the same office) thru MPLS VPN link.

For some reason, I am asked to configure PAT (Many to One NAT)  the IP address of the IP Phones to One IP address.PATing will be configured in my Gateway Router

Is it recommened to PAT the Voice traffic using one IP Address. I believe only One-One NATing is recommended.

Can you please help me in confirming this setup and the best practice

Thanks in advance

SAIRAM

[paolo bevilacqua · Accepted solution]

NAT in IP telephony, especially for phones, is not recommended.

You can try and open the door to much trouble and frustration, or do things properly, and keep IP connectivity unhindered for safe results.

[snarayanaraju]

Thank you very much, Gordon , Paolo

regards

SAIRAM

[paolo bevilacqua]

Thanks for the nice rating and good luck!

[Gordon Ross · Accepted solution]

IP Phones (either SCCP or SIP) won't work with NAT or PAT. If you need to  hide the IP addresses of the phones, use a VPN solution for the phones. Most (all ?) VoIP solutions require the IP traffic to flow without being altered. VPNs handle this by encapsulating the IP traffic into another IP flow (e.g. IPSec), and de-encapsulating it at the other end.

You can't do a plain NAT or PAT for SCCP or SIP, as the IP address & port of the phones is burried inside the IP traffic, and just re-writing the IP header will NOT work. Plus, the SCCP or SIP traffic will also setup RTP UDP streams, so the PAT/NAT box will need to be aware of this too !

Oh, and your NAT/PAT solution will need to handle all the QoS parameters correctly...

Short answer: Don't do it !

GTG