To resolve these issues, locate the point at which RTP packets are not being transmitted in both directions. One-way or no-way voice can begin at a number of points in a packet voice network, as shown:

• IP phone
• Default gateway
• Routing network
• Voice gateway
• Network Address Translation (NAT) or PIX Firewall
• Telco interaction, in particular with audio cut-through

1. Check to see if the IP phone is sending and receiving packets to and from its default gateway.

    

   1. Press the I button twice on a Cisco 7900 series IP phone and check if the RxCnt and TxCnt statistics are incrementing.

       

   2. If the IP phone is not sending RTP packets, it may not have knowledge of its default gateway. Check the DHCP scope of the IP phone to ensure it is providing the correct default gateway to the IP phone.

    

2. Check to see if the default gateway is sending and receiving packets to and from the IP phone.

    

   1. For a single test call, issue the debug voip rtp command on the default gateway. To check connectivity, issue the ping command. An IP phone may not respond to all pings to guard against denial of service attacks.

       

      Note: The debug voip rtp command severely impacts performance and should be used for only single-call debug capture. The command became available in Cisco IOS® Software Releases 12.2(2)XB and 12.2(8)T. It is similar in function to the hidden debug cch323 command. rtp

       

   2. If the default gateway is not sending packets to the IP phone, it may not have routing enabled. Check that the ip routing global command is configured.

       

   3. If packets are being sent and received between the IP phone and its default gateway, the problem lies in the IP routing network or beyond. Continue working to resolve the issue at the voice gateway.

    

3. Check to see if the voice gateway is sending and receiving RTP packets to and from the default gateway.

    

   1. For a single test call, issue the debug voip rtp command on the voice gateway. To check connectivity, issue the ping command.

       

      The ping command allows you to check IP layer connectivity but does not give you an indication of whether RTP packets are traversing in both directions.

      Note: The debug voip rtp command severely impacts performance and should be used for only single-call debug capture. The command became available in Cisco IOS Software Releases 12.2(2)XB, 12.2(8)T, and 12.2(11)T for AS5300, AS5400 and AS5850 access servers.

       

   2. If the voice gateway is not sending and receiving RTP packets to and from the default gateway, there is likely an IP routing problem. Ensure that the ip routing command is enabled. Use the normal techniques for a resolution, including the traceroute and ping commands to resolve a routing problem. The problem can involve a range of issues, including intermittent router hardware failure.

       

   3. If pings are successfully sent and received, the problem may lie with RTP packet transmission. Check for these possibilities:

       

      • If compressed RTP (cRTP) is being used, check that it has been configured at both sides of any WAN link.

         

      • If NAT, or a PIX Firewall, is in the path of the connectivity, check that the appropriate ports are open on the PIX Firewall and that the version of NAT supports Skinny Client Control Protocol (SCCP).

       

   4. Ensure that SCCP default port 2000 is open on the PIX Firewall. If you are using a different port, ensure that the port is open on the PIX Firewall and also that NAT is aware of the port being used. For Cisco IOS NAT, the default port of 2000 is supported by default and the ip nat service skinny tcp port <number> command is required to support other ports.

       

      The minimum software levels required for using NAT and SCCP simultaneously are Cisco IOS Software Release 12.1(5)T for Cisco IOS Software gateways to support SCCP and H.323 version 2 to support NAT. For more information, refer to NAT: Support of IP Phone to Cisco CallManager.

      The minimum software level required for using NAT and SCCP simultaneously on a PIX Firewall is 6.0. For more information, refer to Cisco PIX Firewall Version 6.0.

       

   5. If packets are being sent and received by the voice gateway, continue resolving the issue outside the packet network.

    

4. Check to see if the voice is being cut-through correctly in both directions.

    

   To address problems with audio cut-through, issue the voice rtp send-recv global configuration command on the voice gateway. For more information, refer to Troubleshooting One Way Voice Issues.