This document explains how to force fax calls to use Voice over IP
(VoIP) Fax Relay rather than local hairpinning. This functionality is useful in
a scenario that includes a Primary Rate ISDN (PRI) port accepting voice and fax
calls. The voice calls are directed to IP phones and the fax calls are directed
to Foreign Exchange Station (FXS) ports on the same router.
Local hairpinning of analog calls on a router without a time-division
multiplexing (TDM) bus makes those calls subject to delay on the router
backplane and Digital Signal Processor (DSP) buffers, and therefore unreliable.
VoIP in general, and Fax Relay specifically, overcomes this problem for fax
calls by terminating them directly on the router DSP.
This forced Fax Relay is accomplished when you route the incoming fax
call setup to the Cisco CallManager server, and then immediately redirect it to
the same gateway.
In summary, the gateway now terminates the fax call using Fax Relay on
one leg, establishes a VoIP Fax Relay call between its voice cards routed
through the Cisco CallManager, and then re-establishes the fax call on the FXS
Note: Only the call setup messages pass through the Cisco CallManager.
After the VoIP call is established, data travels directly between the ingress
and egress DSPs on the gateway voice cards.
There are no specific requirements for this document.
The information in this document is based on the software and hardware
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Refer to the
Technical Tips Conventions for more information on document
Use this procedure to configure the Cisco CallManager server to route
the fax calls.
Note: The setup in this document makes use of Cisco CallManager 3.0.
However, the concept is relevant for all versions of Cisco CallManager
including 3.x and 4.x.
Complete these steps to configure the Cisco CallManager server to route
Select Device > Phone >
Add New Phone to create a dummy extension.
In this case, Phone Type Cisco 30 VIP is used.
Insert a dummy MAC address in the MAC address field. For instance,
In the Button Template field, be sure to select a 30 VIP handset
(it has plenty of line appearances) and insert it into the database.
Assume these for the dummy extension (use any numbers that are
available on your system):
line 1 is extension 4444, call forward always to 5555
line 2 is extension 4445, call forward always to 5556
line 3 is extension 4446, call forward always to 5557
line 4 is extension 4447, call forward always to 5558
The Call Forward Always settings route patterns that point back out
to the H.323 gateway, specifically to the FXS ports. This forces the router to
establish a VoIP call. Therefore, it should use Fax Relay to terminate the fax
call on one leg and bridge it to the FXS call leg.
Click on the first line appearance and enter a dummy number in the
Directory Number field. In this example 4444 is used. Then, enter a Forward All
number that points back to the FXS destination pattern. This example uses 5555.
In the VoIP world, route patterns are the equivalent of static
routes. The only difference is that route patterns point to an E.164 number
instead of an IP address. Create and insert a Route Pattern that matches the
forward all number from the dummy extension and direct this to the H.323
gateway with the FXS ports (the H.323 gateway must have been added previously).
In order to do this, go to the Route Plan menu and select Route
Plan > Route Pattern > Add a New
Go back to the Dummy Extension Configuration page and add a new
line number, (for example, 4445) and call forward all numbers (5556). Create a
new Route Pattern that matches the Call Forward All number and points to the
H.323 gateway. Repeat this for each fax line you have.
On the gateway, create these VoIP and plain old telephone server (POTS)
Dial-peer voice 1 voip
!--- Wildcard match for 444X numbers.
Session target ipv4:172.16.1.252
Ip precedence 5
dial-peer voice 5555 pots
dial-peer voice 5556 pots
dial-peer voice 5557 pots
dial-peer voice 5558 pots
You should now be able to receive fax calls on your system.
Use the show voice call summary command to
verify the change of the codec when the fax call is processed by the DSP.
Certain show commands are supported by the
Output Interpreter Tool
(registered customers only)
, which allows you to view an
analysis of show command output.
There is currently no specific troubleshooting information available
for this configuration.