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Fax relay problems on the ICS 7750 using MGCP


Core Issue

There are two types of fax relay services supported on Cisco products:

  • Cisco proprietary fax relay
  • Standards based T.38 fax relay

In order to have fax relay work across a network end-to-end, the components must support compatible versions of fax relay for the particular signaling protocol used.

For more information on fax gateway compatibility, refer to Cisco AVVID Gateway Support for Fax Relay and Fax Pass-Through.


The Cisco Integrated Communications System (ICS) 7750 Multiservice Route Processor (MRP) can be configured to use T.38 fax relay for H.323. However, for Media Gateway Control Protocol (MGCP), Cisco proprietary fax relay is the only supported fax relay protocol.

For more information on fax relay support for the Cisco ICS 7750 MRP, refer to Cisco ICS 7750 MRP Software and Hardware Feature and IOS Version Compatibility.

Cisco CallManager MGCP code does not support T.38. For this reason, the ccm-manager fax protocol cisco command is the default for MGCP on Cisco IOS  Software gateways.

Cisco IOS Software Release 12.2(11)T2 makes the mgcp fax t38 gateway force command available. This provides a way to enable gateway controlled T.38 fax relay between a MGCP gateway and another gateway, even if the capability to use T.38 and Named Signaling Events (NSEs) cannot be negotiated by the MGCP call agent at call setup time. For more information on this command, refer to Update to mgcp fax t38 Command.

Unfortunately, this command is not available on the MRP since the Cisco IOS T-train is not supported. Therefore, make use of T.38 by putting this configuration on the MRP:

voice service voip

fax protocol t38 ls-redundancy 0 hs-redundancy 0

h323 call start fast

This configuration forces the fax relay to use T.38 standards. However, it requires the use of H.323 rather than MGCP.

If you want to continue using MGCP, Cisco proprietary fax relay is required and works fine for Cisco CallManager and Cisco gateways.

Note: The mgcp fax t38 inhibit command is required for operation with Cisco CallManager version 3.1.2c, but is optional for version 3.1.3a and later. This command prevents the MRP or Analog Station Interface (ASI) from sending T.38 fax gateway information to the Cisco CallManager. This command is also required for Cisco Base Transceiver Station (BTS) 10200 versions earlier than 3.0.

For more information on resolving fax relay, refer to Fax Relay Troubleshooting Guide.

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