Voice Port Configuration Guide, Cisco IOS Release 15M&T
PSTN Fallback
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Contents

PSTN Fallback

The PSTN Fallback feature monitors congestion in the IP network and redirects calls to the Public Switched Telephone Network (PSTN) or rejects calls on the basis of network congestion. This feature can also use the ICMP ping mechanism to detect loss of network connectivity and then reroute calls. The fallback subsystem has a network traffic cache that maintains the Calculated Planning Impairment Factor (ICPIF) or delay/loss values for various destinations. Performance is improved because each new call to a well-known destination does not have to wait on a probe to be admitted and the value is usually cached from a previous call.

ICPIF calculates an impairment factor for every piece of equipment along the voice path and then adds them up to get the total impairment value. Refer to International Telecommunication Union (ITU) standard G.113 for more information. The ITU assigns a value to the types of impairment, such as noise, delay, and echo.

Finding Feature Information

Your software release may not support all the features documented in this module. For the latest caveats and feature information, see Bug Search Tool and the release notes for your platform and software release. To find information about the features documented in this module, and to see a list of the releases in which each feature is supported, see the feature information table at the end of this module.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/​go/​cfn. An account on Cisco.com is not required.

Information About PSTN Fallback

Service Assurance Agent

Service Assurance Agent (SAA) is a network congestion analysis mechanism that provides delay, jitter, and packet loss information for the configured IP addresses. SAA is based on a client/server protocol defined on the User Datagram Protocol (UDP). UDP is a connectionless transport layer protocol in the IP protocol stack. UDP is a simple protocol that exchanges datagrams without acknowledgments or guaranteed delivery, requiring that error processing and retransmission be handled by other protocols. The SAA probe packets go out on randomly selected ports from the top end of the audio UDP port range.

The information that the SAA probes gather is used to calculate the ICPIF or delay/loss values that are stored in a fallback cache, where they remain until the cache ages out or overflows. Until an entry ages out, probes are sent periodically for that particular destination. This time interval is user configurable.

With this feature enhancement, you can also configure codes that indicate the cause of the network rejection; for example, packets that are lost or that take too long to be transmitted. A default cause code of 49 displays the message qos-unavail, which means Quality of Service is unavailable.


Note

The Cisco SAA functionality in Cisco IOS software was formerly known as Response Time Reporter (RTR). In the How to Configure PSTN Fallback section, note that the command-line interface still uses the keyword rtr for configuring RTR probes, which are now actually the SAA probes.

Application of PSTN Fallback

The PSTN Fallback feature and enhancement provide the following benefits:

  • Automatically re-routes calls when the data network is congested at the time of the call setup.
  • Enables the service provider to give a reasonable guarantee about the quality of the conversation to its Voice over IP (VoIP) users at the time of call admission.
  • Provides delay, jitter, and packet loss information for the configured IP addresses.
  • Caches call values from previous calls. New calls do not have to wait for probe results before they are admitted.
  • Enables a user-configurable cause code display that indicates the type of call rejection.

Restrictions for PSTN Fallback

The PSTN Fallback feature has the following restrictions:

  • When detecting network congestion, the PSTN fallback feature does nothing to the existing call. It affects only subsequent calls.
  • Only a single ICPIF/delay-loss value is allowed per system.
  • A small additional call setup delay can be expected for the first call to a new IP destination.

Caution

Configuring call fallback active in a gateway creates an SAA jitter probe against other (target) gateways to which the calls are sent. In order for the call fallback active to work properly, the target gateways must have the rtr responder command (in Cisco IOS releases prior to 12.3(14)T) or the ip sla monitor responder command (in Cisco IOS Release 12.3(14)T or later) in their configurations. If one of these commands is not included in the configuration of each target gateway, calls to the target gateway will fail.

How to Configure PSTN Fallback

Configuring Call Fallback to Use MD5 Authentication for SAA Probes

To configure call fallback to use MD5 authentication for SAA probes, use the following commands.

SUMMARY STEPS

    1.    enable

    2.    configure terminal

    3.    call fallback active

    4.    call fallback key-chain name-of-chain


DETAILED STEPS
     Command or ActionPurpose
    Step 1 enable


    Example:
    Router> enable
     

    Enables privileged EXEC mode.

    • Enter your password if prompted.
     
    Step 2 configure terminal


    Example:
    Router# configure terminal
     

    Enters global configuration mode.

     
    Step 3 call fallback active


    Example:
    Router(config)# call fallback active
     

    Enables the PSTN fallback feature to alternate dial peers in case of network congestion.

     
    Step 4 call fallback key-chain name-of-chain


    Example:
    Router(config)# call fallback key-chain sample
     

    Specifies the use of message digest algorithm 5 (MD5) authentication for sending and receiving Service Assurance Agents (SAA) probes.

     

    Configuring Destination Monitoring without Fallback to Alternate Dial Peers

    To configure destination monitoring without fallback to alternate dial peers, use the following commands.

    SUMMARY STEPS

      1.    enable

      2.    configure terminal

      3.    call fallback monitor


    DETAILED STEPS
       Command or ActionPurpose
      Step 1 enable


      Example:
      Router> enable
       

      Enables privileged EXEC mode.

      • Enter your password if prompted.
       
      Step 2 configure terminal


      Example:
      Router# configure terminal
       

      Enters global configuration mode.

       
      Step 3 call fallback monitor


      Example:
      Router(config)# call fallback monitor
       

      Enables the monitoring of destinations without fallback to alternate dial peers.

       

      Configuring Call Fallback Cache Parameters

      To configure the call fallback cache parameters, use the following commands.

      SUMMARY STEPS

        1.    enable

        2.    configure terminal

        3.    call fallback cache-size number

        4.    call fallback cache-timeout seconds

        5.    clear call fallback cache [ip-address]


      DETAILED STEPS
         Command or ActionPurpose
        Step 1 enable


        Example:
        Router> enable
         

        Enables privileged EXEC mode.

        • Enter your password if prompted.
         
        Step 2 configure terminal


        Example:
        Router# configure terminal
         

        Enters global configuration mode.

         
        Step 3 call fallback cache-size number


        Example:
        Router(config)# call fallback cache-size 5
         

        Specifies the call fallback cache size.

         
        Step 4 call fallback cache-timeout seconds


        Example:
        Router(config)# call fallback cache-timeout 300
         

        Specifies the time after which the cache entry is purged, in seconds. Default: 600.

         
        Step 5 clear call fallback cache [ip-address]


        Example:
        Router(config)# clear call fallback cache 10.1.1.1
         

        Clears the current ICPIF estimates for all IP addresses or a specific IP address in the cache.

         

        Configuring Call Fallback Jitter-Probe Parameters

        To configure call fallback jitter-probe parameters, use the following commands.

        SUMMARY STEPS

          1.    enable

          2.    configure terminal

          3.    call fallback jitter-probe num-packets number-of-packets

          4.    call fallback jitter-probe precedence precedence

          5.    call fallback jitter-probe priority-queue


        DETAILED STEPS
           Command or ActionPurpose
          Step 1 enable


          Example:
          Router> enable
           

          Enables privileged EXEC mode.

          • Enter your password if prompted.
           
          Step 2 configure terminal


          Example:
          Router# configure terminal
           

          Enters global configuration mode.

           
          Step 3 call fallback jitter-probe num-packets number-of-packets


          Example:
          Router(config)# call fallback jitter-probe num-packets 10
           

          Specifies the number of packets for jitter. Default: 15.

           
          Step 4 call fallback jitter-probe precedence precedence


          Example:
          or


          Example:
                      
              call fallback jitter-probe dscp dscp-number


          Example:
          Router(config)# call fallback jitter-probe precedence 2


          Example:
          or


          Example:
          Router(config)# call fallback jitter-probe dscp 2
           

          Specifies the treatment of the jitter-probe transmission. Default: 2.

          Specifies the differentiated services code point (dscp) packet of the jitter-probe transmission.

          Note   

          The call fallback jitter-probe precedence command is mutually exclusive with the call fallback jitter-probe dscp command. Only one of these command can be enabled on the router. Usually, the call fallback jitter-probe precedence command is enabled. When the call fallback jitter-probe dscp command is configured, the precedence value is replaced by the DSCP value. To disable DSCP and restore the default jitter probe precedence value, use the no call fallback jitter-probe dscpcommand.

           
          Step 5 call fallback jitter-probe priority-queue


          Example:
          Router(config)# call fallback jitter-probe priority-queue
           

          Assigns a priority to the queue for jitter probes.

           

          Configuring Call Fallback Probe-Timeout and Weight Parameters

          To configure call fallback probe-timeout and weight parameters, use the following commands.

          SUMMARY STEPS

            1.    enable

            2.    configure terminal

            3.    call fallback probe-timeout seconds

            4.    call fallback instantaneous-value-weight percent


          DETAILED STEPS
             Command or ActionPurpose
            Step 1 enable


            Example:
            Router> enable
             

            Enables privileged EXEC mode.

            • Enter your password if prompted.
             
            Step 2 configure terminal


            Example:
            Router# configure terminal
             

            Enters global configuration mode.

             
            Step 3 call fallback probe-timeout seconds


            Example:
            Router(config)# call fallback probe-timeout 20
             

            Sets the timeout for an SAA probe, in seconds. Default: 30.

             
            Step 4 call fallback instantaneous-value-weight percent


            Example:
            Router(config)# call fallback instantaneous-value-weight 50
             

            Configures the call fallback subsystem to take an average from the last two probes registered in the cache for call requests:

            • percent --Instantaneous value weight, expressed as a percentage. Range: 0 to 100. Default: 66.
             

            Configuring Call Fallback Threshold Parameters

            To configure call fallback threshold parameters, use the following commands.

            SUMMARY STEPS

              1.    enable

              2.    configure terminal

              3.    call fallback threshold delay delay-value loss loss-value


            DETAILED STEPS
               Command or ActionPurpose
              Step 1 enable


              Example:
              Router> enable
               

              Enables privileged EXEC mode.

              • Enter your password if prompted.
               
              Step 2 configure terminal


              Example:
              Router# configure terminal
               

              Enters global configuration mode.

               
              Step 3 call fallback threshold delay delay-value loss loss-value


              Example:
              or


              Example:
                          
              call fallback threshold icpif threshold-value


              Example:
              Router(config)# call fallback threshold delay 100 loss 150


              Example:
              or


              Example:
              Router(config)# call fallback threshold icpif 100
               

              Specifies fallback threshold to use packet delay and loss values. No defaults.

              Note   

              The amount of delay set by the call fallback threshold delay loss command should not be more than half the amount of the time-to-wait value set by the call fallback wait-timeout command; otherwise the threshold delay will not work correctly. Because the default value of the call fallback wait-timeout command is set to 300 milliseconds, you can configure a delay of up to 150 milliseconds for the call fallback threshold delay loss command. If you want to configure a higher threshold, the time-to-wait delay has to be increased from its default (300 milliseconds) using the call fallback wait-timeout command.

              Specifies fallback threshold to use the Calculated Planning Impairment Factor (ICPIF) threshold for network traffic.

               

              Configuring Call Fallback Wait-Timeout

              To configure the call fallback wait-timeout parameters, use the following commands:

              SUMMARY STEPS

                1.    enable

                2.    configure terminal

                3.    call fallback wait-timeout milliseconds


              DETAILED STEPS
                 Command or ActionPurpose
                Step 1 enable


                Example:
                Router> enable
                 

                Enables privileged EXEC mode.

                • Enter your password if prompted.
                 
                Step 2 configure terminal


                Example:
                Router# configure terminal
                 

                Enters global configuration mode.

                 
                Step 3 call fallback wait-timeout milliseconds


                Example:
                Router(config)# call fallback wait-timeout 200
                 

                Configures the waiting timeout interval for a response to a probe in milliseconds. Default: 300 milliseconds.

                Note   

                The time-to-wait period set by the call fallback wait-timeout command should always be greater than or equal to twice the amount of the threshold delay time set by the call fallback threshold delay loss command; otherwise the probe will fail. The delay configured by the call fallback threshold delay loss command corresponds to a one-way delay, whereas the time-to-wait period configured by the call fallback wait-timeout command corresponds to a round-trip delay. The threshold delay time should be set at half the value of the time-to-wait value.

                 

                Configuring VoIP Alternate Path Fallback SNMP Trap

                The VoIP Alternate Path Fallback SNMP Trap feature adds a Simple Network Management Protocol (SNMP) trap generation capability. This feature is built on top of the fallback subsystem to provide an SNMP notification trap when the fallback subsystem redirects or rejects a call because a network condition has failed to meet the configured threshold. The SNMP trap provides VoIP management status MIB information without flooding management systems with unnecessary messages about call status by triggering only when a call has been redirected to the public switched telephone network (PSTN) or the alternative IP port. A call can be rejected because of a network problem such as loss of WAN connection, delay, packet loss, or jitter. This feature supports only VoIP signaling protocol with H.323 in this release.

                This feature has to be configured on the originating gateway and the terminating gateway. To configure the SNMP trap parameters, use the following commands:

                SUMMARY STEPS

                  1.    enable

                  2.    configure terminal

                  3.    call fallback active

                  4.    snmp-server enable traps voice fallback


                DETAILED STEPS
                   Command or ActionPurpose
                  Step 1 enable


                  Example:
                  Router> enable
                   

                  Enables privileged EXEC mode.

                  • Enter your password if prompted.
                   
                  Step 2 configure terminal


                  Example:
                  Router# configure terminal
                   

                  Enters global configuration mode.

                   
                  Step 3 call fallback active


                  Example:
                  Router(config)# call fallback active
                   

                  Enables the PSTN fallback feature to alternate dial peers in case of network congestion.

                   
                  Step 4 snmp-server enable traps voice fallback


                  Example:
                  Router(config)# snmp-server enable traps voice fallback
                   

                  Configures the SNMP trap parameters.

                   

                  What to Do Next

                  Configure the rtr responder command on the terminating voice gateway. If the rtr responder is enabled on the terminating gateway, the terminating gateway responds to the probe request when the originating gateway sends an Response Time Report (RTR) probe to the terminating gateway to check the network conditions.

                  Configuring Call Fallback Map Parameters

                  The call fallback map command option provides a target network summary/consolidation mode. For example, if there are four individual voice gateway routers connected together on a remote LAN via a separate LAN-to-WAN access router, the map option allows a single probe to be sent to the single remote WAN access router (instead of having to maintain separate probes for each of the four voice gateway routers’ IP addresses). Because the remote access and voice gateway routers are connected together on the same remote LAN, the probes to the access router returns similar results to probes to the individual voice gateway routers.

                  To configure call fallback map parameters, use the following commands.

                  SUMMARY STEPS

                    1.    enable

                    2.    configure terminal

                    3.    Do one of the following:

                    • call fallback map map target ip-address address-list ip-address1 ip-address2 ... ip-address7
                    • call fallback map map target ip-address subnet ip-network netmask


                  DETAILED STEPS
                     Command or ActionPurpose
                    Step 1 enable


                    Example:
                    Router> enable
                     

                    Enables privileged EXEC mode.

                    • Enter your password if prompted.
                     
                    Step 2 configure terminal


                    Example:
                    Router# configure terminal
                     

                    Enters global configuration mode.

                     
                    Step 3Do one of the following:
                    • call fallback map map target ip-address address-list ip-address1 ip-address2 ... ip-address7
                    • call fallback map map target ip-address subnet ip-network netmask
                     

                    Specifies the call fallback router to keep a cache table (by IP addresses) of distances for several destination peers sitting behind the router.

                    • map --Fallback map. Range is from 1 to 16. There is no default.
                    • target ip-address --Target IP address.
                    • ip-address1 ip-address2 ... ip-address7 --Lists the IP addresses that are kept in the cache table. The maximum number of IP addresses is seven.

                    Specifies the call fallback router to keep a cache table (by subnet addresses) of distances for several destination peers sitting behind the router.

                     

                    Configuring ICMP Pings to Monitor IP Destinations

                    This capability is enabled to monitor the IP destinations in a VoIP network, which may not support RTR. This monitoring is referred to as ICMP pinging. Based on the RTR or ICMP pinging, results change the operational state of the dial-peer. The configurations described in this section also provide support for monitoring the following session targets configured under a VoIP dial-peer:

                    • DNS
                    • IP version 4
                    • SIP-server
                    • enum

                    To configure call-fallback monitor probes to ping IP destinations, complete one of the following tasks:

                    Dial Peer Configuration

                    To configure dial-peer parameters to use ICMP pings to monitor IP destinations, complete this task. This configuration applies only to VoIP dial peers.

                    SUMMARY STEPS

                      1.    enable

                      2.    configure terminal

                      3.    dial-peer voice tag voip

                      4.    call fallback [icmp-ping| rtr]

                      5.    monitor probe {icmp-ping| rtr} [ip address]


                    DETAILED STEPS
                       Command or ActionPurpose
                      Step 1 enable


                      Example:
                      Router> enable
                       

                      Enables privileged EXEC mode.

                      • Enter your password if prompted.
                       
                      Step 2 configure terminal


                      Example:
                      Router# configure terminal
                       

                      Enters global configuration mode.

                       
                      Step 3 dial-peer voice tag voip


                      Example:
                      Router(config)# dial-peer voice 10 voip
                       

                      Enters dial peer configuration mode, specifies the method of voice encapsulation, and defines a particular dial peer:

                      tag --Digits that define a particular dial peer. Range is from 1 to 2147483647.

                       
                      Step 4 call fallback [icmp-ping| rtr]


                      Example:
                      Router(config-dial-peer)# call fallback icmp-ping
                       

                      Configures dial-peer parameters for pings to IP destinations:

                      • icmp-ping --Uses ICMP pings to monitor the IP destinations.
                      • rtr --Uses RTR probes to monitor the session target and update the status of the dial peer. RTR probes are the default.
                      Note   

                      If this call fallback icmp-ping command is not entered, the call fallback active command in global configuration is used for measurements. If this call fallback icmp-ping command is entered, these values override the global configuration. One of these two commands must be in effect before the monitor probe icmp-pingcommand can be used. If neither of call fallback commands is in effect, the monitor probe icmp-ping command will not work properly.

                       
                      Step 5 monitor probe {icmp-ping| rtr} [ip address]


                      Example:
                      Router(config-dial-peer)# monitor probe icmp-ping
                       

                      Enables dial-peer status changes based on the result of the probe:

                      • icmp-ping --Uses ICMP ping as the method for the probe.
                      • rtr --Uses RTR as the method for the probe.

                      ip address --IP address of the destination to be probed. If no IP address is specified, the IP address is read from the session target.

                       

                      Global Configuration

                      To configure global parameters to use ICMP pings to monitor IP destinations, complete this task.

                      SUMMARY STEPS

                        1.    enable

                        2.    configure terminal

                        3.    call fallback active [icmp-ping| rtr]

                        4.    call fallback icmp-ping [count number] [codec type] | size bytes] interval seconds [loss number] [timeout milliseconds]


                      DETAILED STEPS
                         Command or ActionPurpose
                        Step 1 enable


                        Example:
                        Router> enable
                         

                        Enables privileged EXEC mode.

                        • Enter your password if prompted.
                         
                        Step 2 configure terminal


                        Example:
                        Router# configure terminal
                         

                        Enters global configuration mode.

                         
                        Step 3 call fallback active [icmp-ping| rtr]


                        Example:
                        Router(config)# call fallback active icmp-ping
                         

                        Configures global parameters for pings to IP destinations:

                        • icmp-ping --Uses ICMP pings to monitor the IP destinations.
                        • rtr --Uses RTR probes to monitor the IP destinations. RTR probes are the default.
                        Note   

                        The call fallback active icmp-ping command must be entered before the call fallback icmp-ping command can be used. If you do not enter this command first, the call fallback icmp ping command will not work properly.

                         
                        Step 4 call fallback icmp-ping [count number] [codec type] | size bytes] interval seconds [loss number] [timeout milliseconds]


                        Example:
                        Router(config)# call fallback icmp ping codec g729 interval 10 loss 10
                         

                        Configures the parameters for ICMP pings:

                        • count --Number of ping packets to be sent to the destination IP address. Default is 5.
                        • codec --Codec type for deciding the ping packet size.
                        • type --Acceptable codec types are g711a, g711u, g729, and g729b.
                        • size --Size (in bytes) of the ping packet. Default is 32.
                        • interval --Time (in seconds) between ping packet sets. Default is 5. This value should be more than the timeout value.
                        • loss --Threshold packet loss, expressed as a percentage. Default is 20.
                        • timeout --Timeout (in milliseconds) for the echo packets. Default is 500.
                         

                        Voice Port Configuration

                        To configure voice-port parameters to use ICMP pings to monitor IP destinations, complete this task.

                        SUMMARY STEPS

                          1.    enable

                          2.    configure terminal

                          3.    voice-port slot / port

                          4.    busyout monitor probe icmp-ping ip address [codec type | size bytes][loss percent]


                        DETAILED STEPS
                           Command or ActionPurpose
                          Step 1 enable


                          Example:
                          Router> enable
                           

                          Enables privileged EXEC mode.

                          • Enter your password if prompted.
                           
                          Step 2 configure terminal


                          Example:
                          Router# configure terminal
                           

                          Enters global configuration mode.

                           
                          Step 3 voice-port slot / port


                          Example:
                          Router(config)# voice-port 1/0
                           

                          Enters voice-port configuration mode and identifies the slot and port where the configuration parameters take effect.

                          Note   

                          The syntax for this command varies by platform. For more information, see the Cisco IOS Voice Command Reference

                           
                          Step 4 busyout monitor probe icmp-ping ip address [codec type | size bytes][loss percent]


                          Example:
                          Router(config-voiceport)# busyout monitor probe 10.1.1.1 g711u loss 10 delay 2000
                           

                          Specifies the parameters for ICMP pings for monitoring under voice-port configuration:

                          • ip address --IP address of the destination to which the ping is sent.
                          • codec --(Optional) Codec type for deciding the ping packet size.
                          • type --Acceptable codec types are g711a, g711u, g729, and g729b.
                          • size --(Optional) Size (in bytes) of the ping packet. Default is 32.

                          loss --(Optional) Threshold packet loss, expressed as a percentage. Default is 20.

                           

                          Voice Class Configuration

                          To configure voice-class parameters to use ICMP pings to monitor IP destinations, complete this task.

                          SUMMARY STEPS

                            1.    enable

                            2.    configure terminal

                            3.    voice class busyout tag

                            4.    busyout monitor probe icmp-ping ip address [codec type | size bytes][loss percent]


                          DETAILED STEPS
                             Command or ActionPurpose
                            Step 1 enable


                            Example:
                            Router> enable
                             

                            Enables privileged EXEC mode.

                            • Enter your password if prompted.
                             
                            Step 2 configure terminal


                            Example:
                            Router# configure terminal
                             

                            Enters global configuration mode.

                             
                            Step 3 voice class busyout tag


                            Example:
                            Router(config)# voice class busyout 10
                             

                            Creates a voice class for local voice busyout functions:

                            tag --Unique identification number assigned to one voice class. Range is 1 to 10000.

                             
                            Step 4 busyout monitor probe icmp-ping ip address [codec type | size bytes][loss percent]


                            Example:
                            Router(config-class)# busyout monitor probe icmp-ping 10.1.1.1 codec g729b size 32
                             

                            Configures the parameters for ICMP pings for monitoring under voice-port:

                            • ip address --IP address of the destination to which the ping is sent.
                            • codec --(Optional) Codec type for deciding the ping packet size.
                            • type --Acceptable codec types are g711a, g711u, g729, and g729b.
                            • size --(Optional) Size (in bytes) of the ping packet. Default is 32.
                            • loss --(Optional) Threshold packet loss, expressed as a percentage. Default is 20.
                             

                            How to Verify and Monitor the PSTN Fallback Feature

                            Verifying PSTN Fallback Configuration

                            The show commands in this section can be used to display statistics and configuration parameters to verify the operation of the PSTN Callback feature:

                            • show running-config --Displays the contents of the currently running configuration file to see if the new feature is configured.
                            • show call history voice --Displays the call history table for voice calls and verify call fallback, call delay, and call loss parameters.
                            • show call fallback cache --Displays the current Calculated Planning Impairment Factor (ICPIF) estimates for all IP addresses in the call fallback cache.
                            • show call fallback config --Displays the current configuration.
                            • show call fallback stats --Displays the call fallback statistics.

                            Monitoring and Maintaining PSTN Fallback

                            Use the following commands to monitor and maintain the PSTN Fallback feature:

                            • clear call fallback cache --Clears the current ICPIF estimates for all IP addresses in the cache.
                            • clear call fallback stats --Clears the call fallback statistics.
                            • debug call fallback detail --Displays details of VoIP call fallback.
                            • debug call fallback probes --Displays details of voice fallback probes.
                            • test call fallback probe ip-address --Tests a probe to a particular IP address and displays the ICPIF SAA values.
                            • debug snmp packets --Displays information about every Simple Network Management Protocol (SNMP) packet sent or received by the router.

                            What To Do Next

                            The Configuring ICMP Pings to Monitor IP Destinations describes the mechanism whereby a dial-peer becomes temporarily disabled because of poor SAA/RTR probe results (for example, ICPIF, jitter, or loss), or because of failure of the ICMP ping test. When this occurs, the normal alternate dial-peer selection process (hunting) is triggered to search for an alternate dial-peer that represents an alternate route.

                            The global configuration voice hunt command controls whether hunting (continue to look or "hunt" for an alternate dial-peer match) occurs, based on the specific cause code that describes why the initial dial-peer path failed. Hunting is usually appropriate if the cause code indicates network congestion, but usually inappropriate if the failure cause code indicates that the called user is actually busy. Even if an alternate path is taken to reach the called user, and if the user is actually busy, the user will be busy regardless of which path is used.

                            For more information about the voice hunt command, see the Cisco IOS Voice Command Reference .