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Wireshark not seeing Cisco 7945G VOIP traffic as RTP

Posted on 2011-09-20
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Last Modified: 2012-08-13
I put the port on my Cisco 7945G phone into promiscuous mode so that my laptop see all traffic to/from the phone.  When I make a call I see a big stream of traffic from the phone to the voice gateway.  See example packet below.  This must be RTP traffic.  But wireshark is not classifying as such just seeing it as UDP traffic.   I am sure that these UDP packets are carrying voice.  When I go into Telephony/RTP Streams Wireshark says it sees no RTP streams.  Telephony/Voip Calls likewise yields nothing.  Why is wireshark not perceiving the voice traffic as expected?  Thanks!

336      09:56:36.912318      10.10.34.50      10.10.36.6      UDP      Source port: 28176  Destination port: 16486
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Question by:amigan_99
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José Méndez earned 500 total points
ID: 36570298
Not sure why honestly, but you can right click > Decode as > RTP and it will clear the screen if there are no rtp packets, or show them all decoded if truly rtp.
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by:amigan_99
ID: 36570341
Yes!  So now wireshark sees those as G.729.  That reminds me I had put my own phone to G.729 as an experiment way back.  So if I change to G.711 - Telephony/Voice calls would let me listen to the stream?  Or is that another program?  A huge help.
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by:José Méndez
ID: 36570369
Yes, G.711 will allow wireshark to transcode the information. I think you can use Asterisk to transcode from G.729 to wav as explained here: http://www.experts-exchange.com/expertsZone.jsp

Regards,
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