MTP Required on G.729 for RFC 2833?

[zangelo_vbar]

I have a call that passes through the following devices:

 
Cisco 7942 --(SCCP)--> Cisco CME (28xx ISR) --(SIP Trunk)--> UCM 7.0 --(MGCP)--> Cisco 28xx ISR --> PRI

I'm not able to get DTMF to relay reliably when using G.729 (Annex A or B).  In the Cisco UCM trace, I see "No more MTP resources available."  I'm not sure why an MTP resource would be required for DTMF since it should be using RFC 2833 throughout.

The dial-peer on CME is:

dial-peer voice 2 voip
 destination-pattern ....T
 session protocol sipv2
 session target ipv4:10.0.20.60
 dtmf-relay rtp-nte

The MGCP configuration on the PRI gateway is:

mgcp
mgcp call-agent 10.0.20.60 2427 service-type mgcp version 0.1
mgcp dtmf-relay voip codec all mode nte-ca
mgcp rtp unreachable timeout 1000 action notify
mgcp modem passthrough voip mode nse
mgcp package-capability rtp-package
no mgcp package-capability res-package
mgcp package-capability sst-package
no mgcp package-capability fxr-package
mgcp package-capability pre-package
no mgcp timer receive-rtcp
mgcp sdp simple
mgcp rtp payload-type g726r16 static

I've tried both NTE modes on the gateway, neither seem to work.  Any assistance would be greatly appreciated.

[zangelo_vbar]

It looks like my CUCM configuration might be the problem.  Is there a service parameter or something that must be enabled before CUCM will negotiate RFC 2833?  I'm not seeing any telephony-event codecs offered in the SDP no matter where the call is originating and regardless of whether or not it's hitting the PRI gateway.

[Steven Holl · Accepted solution]

CM will need to invoke an MTP, unless you configure the MGCP gateway for RTP-NTE with:

mgcp dtmf-relay voip codec all mode nte-gw

If you configure that and flap the MGCP process and click 'reset gateway' from CM, CM will have that endpoint registered with an RTP-NTE cap, so since it is RTP-NTE on both sides, RTP-NTE will flow from CME directly to the MGCP gateway.  If this still doesn't work, collect CM traces and these debugs for a failure, maybe there's a payload type issue.

MGCP gateway:

debug mgcp pack

debug voip rtp sess name

SIP gateway:

debug ccsip all

debug voip rtp sess name

The other option is to configure MGCP NTFY:

mgcp dtmf-relay voip codec all mode out-of-band

With this configuration, you need to have the MRG on the SIP trunk containing an IOS SW MTP for g.729 so that it can get invoked and terminate RTP-NTE and notify CM via SCCP that it needs to send an MGCP NTFY to the MGCP gateway.

For both configurations, make sure you configure 'rtp-nte' on the SIP trunk configuration in CM.

[Steven Holl]

It looks like my CUCM configuration might be the problem.  Is there a service parameter or something that must be enabled before CUCM will negotiate RFC 2833?  I'm not seeing any telephony-event codecs offered in the SDP no matter where the call is originating and regardless of whether or not it's hitting the PRI gateway.

CM should be offering it as long as:

* You have RTP-NTE in CM under that SIP trunk.  Reset the trunk from CM if you haven't yet after setting this.

* The MGCP gateway is configured for RTP-NTE *or* the MGCP gateway is configured for NTFY but CM is able to invoke an MTP.

[zangelo_vbar]

Thanks for your help.  I was able to get this working last night during a maintenance window.  Even though I tried resetting the gateway through

UCM yesterday, the nte-gw setting didn't take until I bounced mgcp on the router.