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Cisco IP Telephony Troubleshooting Guide for Cisco CallManager Release 3.0(1)
© 2000 Cisco Systems, Inc. 27
enabled Cisco IOS Gateway, then check the configuration carefully. IP routing must be enabled
(look at the configuration to make sure that “no ip routing” is not found near the beginning of the
configuration). Also, make sure that if you’re using RTP header compression to save bandwidth
across the WAN, that it is enabled on each router carrying voice traffic that attaches to the WAN
circuit. There should not be a situation where the RTP header is compressed on one end but
cannot be de-compressed on the other side of the WAN. A sniffer is a very useful tool when
troubleshooting one-way audio problems, since you can then verify that the phone or gateway is
actually sending or receiving packets. Diagnostic Call Detail Records (CDRs) are useful for
determining if a call is experiencing one-way audio since they log transmitted and received
packets (see “Lost or Distorted Audio” section). You can also press the i button twice quickly
on a Cisco IP Phone 7960 during an active call to view details about transmitted and received
packets.
Note: When a call is muted, no packets will be transmitted from the phone that has pressed the
mute button. The Hold button stops the audio stream, so no packets are sent in either direction.
When the Hold button is released, all the packet counters are reset. Remember that Silence
Suppression must be disabled on both devices for the TX and RX counters to stay equal.
Disabling Silence Suppression system-wide will not affect Cisco IOS Gateways.
MTP and One-Way Audio
If you are using Media Termination Point (MTP) in a call (to support supplementary services
such as hold and transfer with H.323 devices that do not support H.323 version 2), check to see if
the MTP allocated is working correctly. Cisco IOS routers support H.323 version 2 beginning in
release 11.3(9)NA and 12.0(3)T. Starting with Cisco IOS release 12.0(7)T, the optional H.323
Open/Close LogicalChannel is supported, so that software-based MTP is no longer required for
supplementary services.
The MTP device, as well as Conference Bridge and Transcoder, will bridge two or more audio
streams. If the MTP, Conference Bridge, or Transcoder is not working properly, one-way audio
or audio loss might be experienced. Shut down MTP to find out if MTP is causing the problem.
Phone Resets
Phones will power cycle or reset for two reasons: 1) TCP failure connecting to
Cisco CallManager, or 2) failure to receive an acknowledgement to the phone’s KeepAlive
messages.
Steps for troubleshooting phone resets:
1. Check the phones and gateways to ensure that you are using the latest software loads.
2. Check CCO (Cisco Connection Online at www.cisco.com) for the latest software loads, new
patches, or release notes relating to the problem.
3. Check the Event Viewer for instances of phone(s) resetting. Phone resets are considered
Information events, as shown in the following illustration.