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A motor noise is experienced on VoIP calls across Frame Relay if cRTP and fast switching are enabled


Core Issue

If VoIP over Frame Relay (VoIPoFR) calls are made between a pair of voice gateways and fast switched Real-Time Protocol (RTP) header-compression is used on the Permanent Virtual Circuit (PVC) between the two routers, both users will hear garbage audio and a ringback tone sounding like a motor. The problem can occur with all codec selections.

For more information, refer to Cisco bug ID CSCdw73527.

Note: This Cisco bug ID has been seen on other router platforms, such as the 1750 and the 2600 and 3600 series routers. The common component is Cisco IOS  Software version 12.2(7) and VoIPoFR. Symptoms include one-way audio and the garbage audio originally described.


To resolve Cisco bug ID CSCdw76315, issue the new ip rtp coalesce global configuration command. This command also resolves Cisco bug ID CSCdw73527. It will not increase the CPU load appreciably above normal fast switched RTP header-compression levels.

To resolve this issue, perform these steps:

  1. Do not use compressed RTP (cRTP) over the PVC. Fast switching on the Frame Relay interfaces by itself is insufficient to trigger the problem.    

  2. Disable fast switching on the Frame Relay interfaces by issuing the no ip route-cache Command-Line Interface (CLI) command.

For more sample voice quality symptoms, refer to Recognizing and Categorizing Symptoms of Voice Quality Problems.