SIP and SCCP phones NAT behind the same public IP address

Unanswered Question
Apr 3rd, 2009

Hi All,

Wondering if anyone has come across this before? We have a situation where two phones, both 7960s, one with SIP and one with SCCP software reside behind a Cisco 2811 doing NAT. When a call is made from the SIP extension to the SCCP extension (or the other way from SCCP to SIP) audio is one way (the SCCP phone can't hear the SIP phone). Pressing the '?' button on the phone twice shows no RTP received on the SCCP phone. On closer inspection this is because SIP phone is sending it's RTP to the router's public address, as instructed by the router. The router returns an ICMP Destination unreachable (Port unreachable) to the SIP phone. So it would appear SIP ALG NAT isn't working correctly for calls between SIP and SCCP endpoints.

RTP flow:

SCCP Phone SIP Phone Public Address

----------------------------------------------

------------------->|

|------------------------>

----------------------------------------------

RTP flow should be:

SCCP Phone SIP Phone

-------------------------

------------------->

<-------------------

-------------------------

Phones register to CUCM 6.1.3 which is routable (no more NAT in the way). Have tried a few different IOSes c2800nm-spservicesk9-mz.124-22.YB1.bin and c2800nm-spservicesk9-mz.124-24.T.bin and the behaviour is the same. Have also tried:

!

ip nat piggyback-support sip sdp-only router 1

!

and:

!

ip nat piggyback-support sip all-messages router 1

!

But still one way audio. Has anyone see this before? Calls SCCP to SCCP are fine, i've only got one SIP phone on there at the moment so can't test SIP to SIP. Calls from the SIP or SCCP phones to gateways outside of the NAT router are fine. Later i'm going to try NATting the SIP phones behind one public address and the SCCP phones behind another on the same router. I'll post back with the results.

Regards,

Jonathan

I have this problem too.
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jonathan.dixon Mon, 04/06/2009 - 02:34

Hi All,

Have since tried putting the phones on separate private networks behind the same router and NATting to separate public addresses, Eg:

SIP phone 10.1.1.10 ---NAT--> 64.64.64.64

SCCP phone 10.1.2.10 ---NAT--> 64.64.64.65

This hasn't worked, RTP from the SCCP phone goes to the internal address of the SIP phone (as instructed by the NAT router) and the audio in this direction is heard. Audio from the SIP phone to the SCCP phone is sent to the public NAT address of the router (as instructed by the NAT router) and no audio is heard on the SCCP phone. It's the same regardless of which phone initiates the call.

To satisfy my curiosity I'm going to put another SIP phone behind the NAT router and see what happens for SIP to SIP internal calls.

Not sure where to go from here, may end up with two routers, HSRP between them with the master for SIP on one router and the master for SCCP on the other.

Regards,

Jonathan

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