Voice class codec issue

Question

Hi,
I have a sip trunk terminating on a CUBE. On the CUBE, I hard-code the dial-peer 211 with G.729 codec. On an inbound call to a DID terminating on the IP phone, the call negotiated g.729 and completed successfully. No issues.
Now I created a voice class codec with 2 codecs in the following preferences
voice class codec 1
codec preference 1 ---> g729
codec preference 2 ---> g711
I then apply this to a dial-peer 211.
On an inbound call from the same incoming number to the same DID(same endpoint), this call is now negotiated only at G.711 codec. I verified that I am hitting the same dial-peer by using "show call active voice brief" and checking the pid. It is using dial-peer 211.
My expectation is that the call will still negotiate G.729 and will use G.711 only if the call cannot be completed at G.729.
As a test, I also verified by removing codec preference 2 (i.e.) G.711 from voice class codec 1 and call negotiated at G.729.
BTW, in each scenario, I used the show voice call active compact to verify the call legs and codecs being used.
CUCM version 9.1 and IOS 15.1(4). Any ideas why this odd behavior?
Regards,
K iyer

Answer 1

Here is the catch with CUCM. CUCM always prefers and will use the "best available codec" offered. Therefore, when the gateway forwards the setup to CUCM, it would see g711 as a valid option and would use it. Here is more info on the same:
http://www.cisco.com/en/US/docs/voice_ip_comm/cucm/admin/9_1_1/ccmsys/CUCM_BK_C5565591_00_cucm-system-guide-91_chapter_0101.html#CUCM_RF_RE6237E1_00
Regions
Regions provide capacity controls for Cisco Unified Communications Manager multi-site deployments where you may need to limit the bandwidth for individual calls that are sent across a WAN link, but where you want to use a higher bandwidth for internal calls. Additionally, the system uses regions also for applications that only support a specific codec; for example, an application that only uses G.711. Use regions to specify the maximum transport-independent bit rate that is used for audio and video calls within a region and between regions; in this case, codecs with higher bit rates do not get used for the call.
Cisco Unified Communications Manager prefers codecs with better audio quality. For example, despite having a maximum bit rate of 32 kb/s, G.722.1 sounds better than some codecs with higher bit rates, such as G.711, which has a bit rate of 64 kb/s.
HTHs
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