Loopback Call Routing
Loopback call routing in a Cisco Unified CME system is provided through a mechanism called loopback-dn, which provides a software-based limited emulation of back-to-back physical voice ports connected together to provide a loopback call-routing path for voice calls.
Loopback call routing and loopback-dn restricts the passage of call-transfer and call-forwarding supplementary service requests through the loopback. Instead of passing these requests through, the loopback-dn mechanism attempts to service the requests locally. This allows loopback-dn configurations to be used in call paths where one of the external devices does not support call transfer or call forwarding (Cisco-proprietary or H.450-based). Control messages that request call transfer or call forwarding are intercepted at the loopback virtual port and serviced on the local voice gateway. If needed, this mechanism creates VoIP-to-VoIP call-routing paths.
Loopback call routing may be used for routing H.323 calls to Cisco Unity Express. For information on configuring Cisco Unity Express, see the Cisco Unity Express documentation.
Note A preferred alternative to loopback call routing was introduced in Cisco CME 3.1. This alternative blocks H.450-based supplementary service requests by using the following Cisco IOS commands: no supplementary-service h450.2, no supplementary-service h450.3, and supplementary-service h450.12. For more information, see Configuring Call Transfer and Forwarding.
Use of loopback-dn configurations within a VoIP network should be restricted to resolving critical network interoperability service problems that cannot otherwise be solved. Loopback-dn configurations are intended for use in VoIP network interworking where the alternative would be to make use of back-to-back-connected physical voice ports. Loopback-dn configurations emulate the effect of a back-to-back physical voice-port arrangement without the expense of the physical voice-port hardware. Because digital signal processors (DSPs) are not involved in loopback-dn arrangements, the configuration does not support interworking or transcoding between calls that use different voice codecs. In many cases, use of back-to-back physical voice ports that do involve DSPs to resolve VoIP network interworking issues is preferred, because it introduces fewer restrictions in terms of supported codecs and call flows.
Loopback call routing requires two extensions (ephone-dns) to be separately configured, each as half of a loopback-dn pair. Ephone-dns that are defined as a loopback-dn pair can only be used for loopback call routing. In addition to defining the loopback-dn pair, you must specify preference, huntstop, class of restriction (COR), and translation rules.