Cisco SIP T.38 fax failing (Cisco sends to wrong port)

I am seeing issues with Cisco IOS 12.3(13) on a 5300. I use SIP to try to negotiate a T.38 fax relay session with my equipment.

The call starts as a voice call and Cisco will be sending RTP packets to the correct RTP/UDP port number. Then as we enter fax mode, the re-invite process completes and looks fine. My end sends Cisco the new T.38 port number to use for T.38 packets (it is RTP port plus 2). Cisco ACKs this, but when Cisco starts to send T.38 packets it is sending them to my RTP port instead of my T.38 port. There are some times when I will start sending T.38 packets from my T.38 port to the correct Cisco T.38 port (Cisco uses the same port number for RTP and T.38), and Cisco will see these packets and switch to start sending it's T.38 to my correct T.38 port. Many times Cisco will never switch no matter what I send it, and the fax will fail. It would be best for me if Cisco would start sending T.38 to the negotiated T.38 port immediately after the re-invite process ends.

Using the same Cisco box for H.323 calls, the Cisco will send T.38 packets to the Correct T.38 port immediately, so it will use the T.38 port that it got via H.323 signaling and not depend on any RTP or T.38 packets coming in to make it switch.

I assume that this is a bug in SIP with 12.3 IOS. Is anybody else seeing this?

Also, if you know of any other forums where I might get this question answered, I'd appreciate it.



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This has turned out to be an issue only when the AS5300 has Symmetrical NAT support enabled. This is the case on IOS builds 12.3(13), 12.3(16) and 12.3(15b).

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