loopback-dn command is used to configure two
ephone-dn virtual voice ports as back-to-back-connected voice-port pairs. A
call presented on one side of the loopback-dn pair is reoriginated as a new
call on the opposite side of the loopback-dn pair. The
suffix keywords can be used to manipulate the
original called number that is presented to the incoming side of the
loopback-dn pair to generate a modified called number to use when reoriginating
the call at the opposite side of the loopback-dn pair. For loopback-dn
configurations, you must always configure ephone-dn virtual voice ports as
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 to be used
in VoIP network interworking situations in which the only other 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.
A disadvantage of loopback-dn configurations is that, because digital signal
processors (DSPs) are not involved in a loopback-dn arrangement, the
configuration does not support interworking or transcoding between calls that
use different voice codecs. In many cases, the use of back-to-back physical
voice ports that do use DSPs to resolve VoIP network interworking issues is
preferred, because it introduces fewer restrictions in terms of supported
codecs and call flows. Also, loopback-dns do not support T.38 fax relay.
We recommend that you create the basic ephone-dn configuration for
both ephone-dn entries before configuring the loopback-dn option under each
ephone-dn. The loopback-dn mechanism should be used only in situations where
the voice call parameters for the calls on either side of the loopback-dn use
compatible configurations; for example, compatible voice codec and dual tone
multifrequency (DTMF) relay parameters. Loopback-dn configurations should be
used only for G.711 voice calls.
The loopback-dn arrangement allows an incoming telephone call to be
terminated on one side of the loopback-dn port pair and a new pass-through
outgoing call to be originated on the other side of the loopback-dn port pair.
The loopback-dn port pair normally works with direct cross-coupling of their
call states; the alerting call state on the outbound call segment is associated
with the ringing state on the inbound call segment.
The loopback-dn mechanism allows for call operations (such as call
transfer and call forward) that are invoked for the call segment on one side of
the loopback-dn port pair to be isolated from the call segment that is present
on the opposite side of the loopback-dn port pair. This approach is useful when
the endpoint devices associated with the two different sides have mismatched
call-transfer and call-forwarding capabilities. The loopback-dn arrangement
allows for call-transfer and call-forward requests to be serviced on one side
of the loopback-dn port pair by creating hairpin-routed calls when necessary.
The loopback-dn arrangement avoids the propagation of call-transfer and
call-forward requests to endpoint devices that do not support these functions.
loopback-dn command provides options for
controlling the called-number digits that are passed through from the incoming
side to the outgoing side. The available digits can be manipulated with the
forward keyword defines the number of digits
in the original called number to forward to the other ephone-dn in the
loopback-dn pair. The default is set to forward all digits. The
strip keyword defines the number of leading
digits to be stripped from the original called number before forwarding to the
other ephone-dn in the loopback-dn pair. The default is set to not strip any
strip commands are mutually exclusive and can
be used with any combination of the
prefix keyword defines a string of digits to
add in front of the forwarded number.
suffix keyword is most commonly used to add a
terminating “#” (pound-sign) character to the end of the forwarded number to
indicate that no more digits should be expected. The pound-sign character
indicates to the call-routing mechanism that is processing the forwarded number
that the forwarded number is complete. Providing an explicit end-of-number
character also avoids a situation in which the call-processing mechanism waits
for the interdigit timeout period to expire before routing the call onward
using the forwarded number.
The Cisco IOS command-line interface (CLI) requires that arguments
with character strings that start with the pound-sign (#) character be enclosed
within quotation marks; for example, “#”.
retry keyword is used to suppress a far-end
busy indication on the outbound call segment. Instead of returning a busy
signal to the call originator (on the incoming call segment), a loopback-dn
presents an alerting or ringing tone to the caller and then periodically
retries the call to the final far-end destination (on the outgoing call
segment). This is not bidirectional. To prevent calls from being routed into
the idle outgoing side of the loopback-dn port pair during the idle interval
that occurs between successive outgoing call attempts, configure the outgoing
side of the loopback-dn without a number so that there is no number to match
for the inbound call.
auto-con keyword is used to configure a
premature trigger for a connected state for an incoming call segment while the
outgoing call segment is still in the alerting state. This setup forces the
voice path to open for the incoming call segment and support the generation of
in-band call progress tones for busy, alerting, or ringback. The disadvantage
auto-con keyword is premature opening of the
voice path during the alerting stage and also triggering of the beginning of
billing for the call before the call has been answered by the far end. These
disadvantages should be considered carefully before you use
codec keyword is used to explicitly select
the A-law or mu-law type of G.711 and to provide A-law to mu-law conversion if
needed. Setting the codec type on one side of the loopback-dn forces the
selection of A-law or mu-law for voice packets that are transmitted from that
side of the loopback-dn. To force the A-law or mu-law G.711 codec type for both
voice packet directions, set the codec type on both sides of the loopback-dn.
Loopback-dn configurations are used only with G.711 calls. Other voice codec
types are not supported.
The following example creates a loopback-dn configured with the
Router(config)# ephone-dn 7
Router(config-ephone-dn)# loopback-dn 15 forward 5 prefix 41
The following example creates a loopback-dn that appends the
pound-sign (#) character to forwarded numbers to indicate the end of the
Router(config)# ephone-dn 7
Router(config-ephone-dn)# loopback-dn 16 suffix “#”
The following example shows a loopback-dn configuration that pairs
ephone-dns 15 and 16. An incoming call (for example, from VoIP) to 4085550101
matches ephone-dn 16. The call is then reoriginated from ephone-dn 15 and sent
to extension 50101. Another incoming call (for example, from a local IP phone)
to extension 50151 matches ephone-dn 15. It is reoriginated from ephone-dn 16
and sent to 4085550151.
loopback-dn 16 forward 5 prefix 40855
loopback-dn 15 forward 5