Loopback Call Routing

Information About Loopback Call Routing

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 Configure 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.

Configure Loopback Call Routing

Enable Loopback Call Routing

To enable loopback call-routing, perform the following steps for each ephone-dn that is part of the loopback-dn pair.


Restriction

Loopback-dns do not support T.38 fax relay.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ephone-dn dn-tag
  4. number number [ secondary number] [ no-reg [ both | primary ]]
  5. caller-id { local | passthrough}
  6. no huntstop
  7. preference preference-order [ secondary secondary-order]
  8. cor { incoming | outgoing} cor-list-name
  9. translate { called | calling} translation-rule-tag
  10. loopback-dn dn-tag [ forward number-of-digits | strip number-of-digits ] [ prefix prefix-digit-string ] [ suffix suffix-digit-string ] [ retry seconds] [ auto-con ] [ codec { g711alaw | g711ulaw}]
  11. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Router# configure terminal

Enters global configuration mode.

Step 3

ephone-dn dn-tag

Example:

Router(config)# ephone-dn 15

Enters ephone-dn configuration mode, creates an ephone-dn, and optionally assigns it dual-line status.

  • dn-tag —Unique sequence number that identifies this ephone-dn during configuration tasks. Range is platform- and version-dependent.

Note

 

Ephone-dns used for loopback cannot be dual-line ephone-dns.

Step 4

number number [ secondary number] [ no-reg [ both | primary ]]

Example:

Router(config-ephone-dn)# number 2001

Associates a number with this extension (ephone-dn).

  • number —String of up to 16 digits that represents a telephone or extension number to be associated with this ephone-dn.

  • secondary —(Optional) Allows you to associate a second telephone number with an ephone-dn.

  • no-reg —(Optional) Specifies that this number should not register with the H.323 gatekeeper. The no-reg keyword indicates that only the secondary number should not register. The no-reg both keywords indicate that both numbers should not register, and the no-reg primary keywords indicate that only the primary number should not register.

Step 5

caller-id { local | passthrough}

Example:

Router(config-ephone-dn)# caller-id local

Specifies caller-ID treatment for outbound calls originated from the ephone-dn. The default if this command is not used is as follows. For transferred calls, caller ID is provided by the number and name fields from the outbound side of the loopback-dn. For forwarded calls, caller ID is provided by the original caller ID of the incoming call. Settings for the caller-id block command and translation rules on the outbound side are executed.

  • local —Passes the local caller ID on redirected calls. This is the preferred usage.

  • passthrough —Passes the original caller ID on redirected calls.

Step 6

no huntstop

Example:

Router(config-ephone-dn)# no huntstop

Disables huntstop and allows call hunting behavior for an extension (ephone-dn).

Step 7

preference preference-order [ secondary secondary-order]

Example:

Router(config-ephone-dn)# preference 1

Sets dial-peer preference for an extension (ephone-dn).

  • preference-order —Preference order for the primary number associated with an extension (ephone-dn). Range is 0 to 10, where 0 is the highest preference and 10 is the lowest preference. Default is 0.

  • secondary secondary-order —(Optional) Preference order for the secondary number associated with the ephone-dn. Range is 0 to 10, where 0 is the highest preference and 10 is the lowest preference. Default is 9.

Step 8

cor { incoming | outgoing} cor-list-name

Example:

Router(config-ephone-dn)# cor incoming corlist1

Applies a class of restriction (COR) to the dial peers associated with an extension. COR specifies which incoming dial peer can use which outgoing dial peer to make a call. Each dial peer can be provisioned with an incoming and an outgoing COR list.

For information about COR, see Dial Peer Configuration on Voice Gateway Routers.

Step 9

translate { called | calling} translation-rule-tag

Example:

Router(config-ephone-dn)# translate called 1

Selects an existing translation rule and applies it to a calling number or a number that has been called. This command enables the manipulation of numbers as part of a dial plan to manage overlapping or nonconsecutive numbering schemes.

  • called —Translates the called number.

  • calling —Translates the calling number.

  • translation-rule-tag —Unique sequence number of the previously defined translation rule. Range is 1 to 2147483647.

Note

 

This command requires that you have previously defined appropriate translation rules using the voice translation-rule and rule commands.

Step 10

loopback-dn dn-tag [ forward number-of-digits | strip number-of-digits ] [ prefix prefix-digit-string ] [ suffix suffix-digit-string ] [ retry seconds] [ auto-con ] [ codec { g711alaw | g711ulaw}]

Example:

Router(config-ephone-dn)# loopback-dn 24 forward 15 prefix 415353....

Enables H.323 call transfer and call forwarding by using hairpin call routing for VoIP endpoints that do not support Cisco-proprietary or H.450-based call-transfer and call-forwarding.

  • dn-tag— Unique sequence number that identifies the ephone-dn that is being paired for loopback with the ephone-dn that is being configured. The paired ephone-dn must be one that is already defined in the system.

  • forward number-of-digits —(Optional) Number of digits in the original called number to forward to the other ephone-dn in the loopback-dn pair. Range is 1 to 32. Default is to forward all digits.

  • strip number-of-digits —(Optional) Number of leading digits to be stripped from the original called number before forwarding to the other ephone-dn in the loopback-dn pair. Range is 1 to 32. Default is to not strip any digits.

  • prefix prefix-digit-string —(Optional) Defines a string of digits to add in front of the forwarded called number. Maximum number of digits in the string is 32. Default is that no prefix is defined.

  • suffix suffix-digit-string —(Optional) Defines a string of digits to add to the end of the forwarded called number. Maximum number of digits in the string is 32. Default is that no suffix is defined. If you add a suffix that starts with the pound character (#), the string must be enclosed in quotation marks.

  • retry seconds —(Optional) Number of seconds to wait before retrying the loopback target when it is busy or unavailable. Range is 0 to 32767. Default is that retry is disabled and appropriate call-progress tones are passed to the call originator.

  • auto-con —(Optional) Immediately connects the call and provides in-band alerting while waiting for the far-end destination to answer. Default is that automatic connection is disabled.

  • codec —(Optional) Explicitly forces the G.711 A-law or G.711 mu-law voice coding type to be used for calls that pass through the loopback-dn. This overrides the G.711 coding type that is negotiated for the call and provides conversion from mu-law to A-law, if needed. Default is that Real-Time Transport Protocol (RTP) voice packets are passed through the loopback-dn without considering the G.711 coding type negotiated for the calls.

  • g711alaw— G.711 A-law, 64000 bits per second, for T1.

  • g711ulaw— G.711 mu-law, 64000 bits per second, for E1.

Step 11

end

Example:

Router(config-ephone-dn)# end

Exits to privileged exec mode.

Verify Loopback Call Routing

Procedure

Use the show running-config or show telephony-service ephone-dn command to display ephone-dn configurations.

Configuration Example for Loopback Call Routing

Example for Enabling Loopback Call Routing

The following example uses ephone-dns 15 and 16 as a loopback-dn pair. Calls are routed through this loopback ephone-dn pair in the following way:

  • An incoming call to 4085552xxx enters the loopback pair through ephone-dn 16 and exits the loopback via ephone-dn 15 as an outgoing call to 2xxx (based on the forward 4 digits setting).

  • An incoming call to 6xxx enters the loopback pair through ephone-dn 15 and exits the loopback via ephone-dn 16 as an outgoing call to 4157676xxx (based on the prefix 415767 setting). 

ephone-dn 15
		 number 6...
		 loopback-dn 16 forward 4 prefix 415767
		 caller-id local
		 no huntstop
		!
		ephone-dn 16
		 number 4085552...
		 loopback-dn 15 forward 4
		 caller-id local
		 no huntstop

Feature Information for Loopback Call Routing

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to https://cfnng.cisco.com/. An account on Cisco.com is not required.
Table 1. Feature Information for Loopback Call Routing

Feature Name

Cisco Unified CME Version

Feature Information

Loopback Call Routing

2.0

Loopback call routing was introduced.