SIP MTP

[jeffkushner]

Working with an older CUCM installation that uses SIP.  One of the issues they have is they have a remote location with a T1 serial. The serial is used for both MPLS back to HQ and for access to the SIP provider. I noticed that on the SIP trunk has MTP required.  This is a cube set up so I decided to uncheck this as it seems to me that the conversation must traverse the serial twice; for the MTP and for the actual call.

When I turned it off the calls would complete but no audio so I checked MTP required and all is good, but I'd like to understand why. I have compared it to several other sites that use cube and sip and this option is not checked nor is the early offer mtp option... thx

[Chris Deren]

Sounds like there may be either firewall or routing contraints allowing for the calls to go directly from phones to CUBE/ITSP hence MTP was inserted. My recommendation is not to use MTPs if possible, especially on every call which is what checking the box does.  Is your CUBE setup for "media flow-through" or "media flow-around", you would want the first option.

[Dennis Mink]

collect the debug ccsip message information. verify the SDP's and look for the o=<ip address> line in the SDP this will tell you between which IP addresses it trying to set up the RTP/voice stream. 

 make sure you can route between these two IP addresses and have a firewall open for RTP between the two IP addresses. 

media flow through makes all this a bit easier but is by no means a requirement to get this to work.

[drbabbers]

MTP is covered in good detail the SRND:

https://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab11/collab11/media.html#77886

Especially the magical MTP tickbox on the SIP Trunk :)