Question about RTP ports

[treefiddy]

Hi,

I am setting up a system and planning to use two SIP trunks. One from Telnyx and one from CenturyLink; both are supported trunks.

Telnyx uses RTP ports 16384 to 32768 and CL uses 20000 to 60000 (or so I was told). How does this work with 3CX, being that the default range is 9000 to 10999?

 

[cobaltit]

The beauty of supported trunks is that you just plug in the info and go don't have to worry about these things...

The technical answer is it doesn't matter what ports are used on the provider end. 3CX will request audio on the ports it wants and the provider will request audio on the ports they want. They don't need to match.

 

[treefiddy]

So the port is just being negotiated on each parties' end? Ex. Telnyx would send from 16843 to 9000 on 3CX?

I ask because I've seen posts about needing to change the RTP ports range before and the Centurylink engineer had also mentioned it.

 

[eddv123]

Please post up the string(s) if you have them for reference, you maybe referring to the recent 3CX requirement to increase the audio port range, see here: https://www.3cx.com/community/threads/3cx-phone-system-audio-ports-increased.60235/

As per: https://www.3cx.com/docs/manual/sip-trunks/

All that is required on your local firewall in regards to port forwarding is the SIP port 5060 and audio ports external Port Range : 9000-10999 [UDP]. As long as the firewall checker passes in this respect you should be fine.

 

[treefiddy]

Thanks for the information guys. My questions have been answered.

 

[cobaltit]

So the port is just being negotiated on each parties' end? Ex. Telnyx would send from 16843 to 9000 on 3CX?

Correct. Here is an example of the SDP from a PCAP.

Media Description, name and address (m): audio 9094 RTP/AVP 0 18 101
    Media Type: audio
    Media Port: 9094
    Media Protocol: RTP/AVP
    Media Format: ITU-T G.711 PCMU
    Media Format: ITU-T G.729
    Media Format: DynamicRTP-Type-101

Media Description, name and address (m): audio 34222 RTP/AVP 0 18 101
    Media Type: audio
    Media Port: 34222
    Media Protocol: RTP/AVP
    Media Format: ITU-T G.711 PCMU
    Media Format: ITU-T G.729
    Media Format: DynamicRTP-Type-101

The first SDP is the invite from 3CX out to our trunking platform. 3CX is requesting audion on port 9094. The second SDP is the OK from our trunking platform requesting audio on 34222.