Which ports do Phones offer in SDP with SIP?

[tschafferx]

Hi Cisco community,

scenario: Phone A --> CUCM --> Phone B | Call from Phone A to Phone B

We assue early offer is used.

Phone A places the call and sends the INVITE with its SDP. In that SDP there is also a field that distiguishes the m= audio with a port on which the phone is listening to at a later stage, when the RTP stream is up and running. QUESTION: From which port range does the phone choose its port? The same question applies for Phone B in the 200 OK message.

I read that CUCM uses RTP ports in the range of 16384 - 32767. When do these ports come into play?

Moreover did I see, that I could also specify a start and stop port range in the SIP profile.

It would be great, if someone could explain these topics to me. Thank you in advance.

[Chris Deren]

Yes, by default phone will request to use UDP port from 16384 - 32767 range unless you change it in CUCM service parameters. Keep in mind these are ports used by Cisco and other endpoints or telcos will use other ports. Newer IOS-XE versions also changed the range, so you might see different ports negotiated by CUBEs/GWs.

[tschafferx]

Hi Chris,

thank you for your reply. I couldn't find the respective settings in the service parameters. Could I also change it in the SIP-Profile? There also seems to be an option to tweak it.

[Chris Deren · Accepted solution]

My apologies, those cannot be found in service parameters but SIP profile as you mention.  Yes, you can change them, but why would you want to do that?

[tschafferx]

I don't want to change them. I was just curious as to where the phones get those information from.

That basically means, that the phones store these information and use them when applicable. Correct?

I also wanted to know, when CUCM uses the dynamic RTP ports 16384 - 32767 / UDP. As seen here:

Table 7 Signaling, Media, and Other Communication Between Gateways and Cisco Unified Communications Manager

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/port/9_1_1/CUCM_BK_T2CA6EDE_00_tcp-port-usage-guide-91/CUCM_BK_T2CA6EDE_00_tcp-port-usage-guide-91_chapter_01.html

Maybe if the CUCM is being used as a MTP?

Thank you.