RTP Flow Problem

[mjbright757]

Hey Everyone,

Hoping someone might be able to assist in a very strange problem I am having.  I have roughly 15 sites working off a CUCM 11.5 with 2 subscribers and only 1 phone at one of these sites is having this issue.  What happens is when this phone makes or receives a phone call, the signalling comes from or is sent to the phone's primary subscriber.  But when the RTP actually starts to flow, it is sent to the backup subscriber.  This site uses a SIP Trunk registered to an AudioCodes SBC which connects to the ITSP.  Issue is intermittent and randomly fixes itself for a few days and then comes back.

In looking through the SIP Message Details, the only thing I can find is that in the Invite message sent from the CUCM to the SBC, it has altered the following line to contain the IP address of the Secondary Subscriber.

c=IN IP4 

Original message from phone to CUCM, that line contains the IP address of the phone.  And on a normal working call it contains the IP address of the Primary Subscriber.

What would cause this?

Thanks guys!

[Chris Deren · Accepted solution]

RTP should not flow through any CUCM, sounds like perhaps an MTP is being invoked for specific call flow and software based MTP is being invoked. Which MTP is used would be based on MRG/MRGL configuration on device invoking it, i.e. SIP trunk.

[mjbright757]

Thanks Chris!  That was the issue and what I've been trying to figure out for the longest time.  The environment was previously CUCM9.1 and the previous vendor who managed the system and the SBCs enabled for MTP required.  I'm not sure how, but previously RTP flowed direct between endpoints and SBCs.  When we migrated the environment over to CUCM 11.5 is when we noticed this flow changed.  I've disabled MTP required under the SIP Trunks, verified integration and call flows still work and performed a packet capture and I am now seeing RTP flow between endpoints and SBCs like I did before.  I also verified that CUCM is no longer altering the connection information in the Invite messages.

Thanks for the direction in resolution!