Hi
With the introduction of Cisco IOS Software Release 12.0(5)T, Cisco VoIP gateways support the codec negotiation feature. This feature provides the ability for a Cisco VoIP gateway to connect to other VoIP devices without necessarily knowing which codec is used for a call-setup. Also, this feature allows Cisco VoIP gateways to dynamically adjust to changes on remote devices. As long as the codec used by the remote VoIP device matches the capabilities-list of the Cisco VoIP gateway, the VoIP call is completed. For example, you got location A and B, if you give this voice class codec config in your A location, if the B location uses an outgoing dialpeer which just uses g729r8 in its voip device, then your A location voip device looks first for g711ulaw codec, as it is not there in B location dialpeer, it falls(negotiates) to the second preference codec which is g729r8, and sets up the call. From my understanding, it is related to dialpeer, codecs, callsetup. If you use a dialpeer which is pointing to a fax , which has a codec specified in it, then it is related there as well, but the concept is the same.
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