Table Of Contents
Information About PSTN Fallback
Restrictions for PSTN Fallback
How to Configure PSTN Fallback
Configuring Call Fallback to Use MD5 Authentication for SAA Probes
Configuring Destination Monitoring without Fallback to Alternate Dial Peers
Configuring Call Fallback Cache Parameters
Configuring Call Fallback Jitter-Probe Parameters
Configuring Call Fallback Probe-Timeout and Weight Parameters
Configuring Call Fallback Threshold Parameters
Configuring Call Fallback Wait-Timeout
Configuring VoIP Alternate Path Fallback SNMP Trap
Configuring Call Fallback Map Parameters
Configuring ICMP Pings to Monitor IP Destinations
Dial Peer Configuration of the call fallback icmp-ping and monitor probe Commands
Global Configuration of the call fallback icmp-ping Command
Voice Port Configuration of the busyout monitor probe icmp-ping Command
Voice Class Configuration of the busyout monitor probe icmp-ping Command
How to Verify and Monitor the PSTN Fallback Feature
Verifying PSTN Fallback Configuration
Monitoring and Maintaining PSTN Fallback
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.
Feature Information
Not all commands may be available in your Cisco IOS software release. For release information about a specific command, see the command reference documentation.
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Contents
This document contains the following sections:
•Information About PSTN Fallback
•Restrictions for PSTN Fallback
•How to Configure PSTN Fallback
•How to Verify and Monitor the PSTN Fallback Feature
Information About PSTN Fallback
This section provides the following information about the PSTN Fallback feature:
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
This section contains the following procedures (each identified as either optional or required):
•Configuring Call Fallback to Use MD5 Authentication for SAA Probes (required)
•Configuring Destination Monitoring without Fallback to Alternate Dial Peers (optional)
•Configuring Call Fallback Cache Parameters (optional)
•Configuring Call Fallback Jitter-Probe Parameters (optional)
•Configuring Call Fallback Probe-Timeout and Weight Parameters (optional)
•Configuring Call Fallback Threshold Parameters (optional)
•Configuring Call Fallback Wait-Timeout (optional)
•Configuring VoIP Alternate Path Fallback SNMP Trap (optional)
•Configuring ICMP Pings to Monitor IP Destinations (optional)
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. config terminal
3. call fallback active
4. call fallback key-chain name-of-chain
DETAILED STEPS
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. config terminal
3. call fallback monitor
DETAILED STEPS
Configuring Call Fallback Cache Parameters
To configure the call fallback cache parameters, use the following commands.
SUMMARY STEPS
1. enable
2. config terminal
3. call fallback cache-size number
4. call fallback cache-timeout seconds
5. clear call fallback cache [ip-address]
DETAILED STEPS
Configuring Call Fallback Jitter-Probe Parameters
To configure call fallback jitter-probe parameters, use the following commands.
SUMMARY STEPS
1. enable
2. config terminal
3. call fallback jitter-probe num-packets number-of-packets
4. call fallback jitter-probe precedence precedence
or
call fallback jitter-probe dscp dscp-number5. call fallback jitter-probe priority-queue
DETAILED STEPS
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 weight
DETAILED STEPS
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
or
call fallback threshold icpif threshold-valueDETAILED STEPS
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
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
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. call fallback map map target ip-address address-list ip-address1 ip-address2 ... ip-address7
or
call fallback map map target ip-address subnet ip-network netmaskDETAILED STEPS
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 of the call fallback icmp-ping and monitor probe Commands
•Global Configuration of the call fallback icmp-ping Command
•Voice Port Configuration of the busyout monitor probe icmp-ping Command
•Voice Class Configuration of the busyout monitor probe icmp-ping Command
Dial Peer Configuration of the call fallback icmp-ping and monitor probe Commands
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
Global Configuration of the call fallback icmp-ping Command
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 number] interval number [loss number] [timeout value]
DETAILED STEPS
Voice Port Configuration of the busyout monitor probe icmp-ping Command
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 Action PurposeStep 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 of the busyout monitor probe icmp-ping Command
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
How to Verify and Monitor the PSTN Fallback Feature
This section provides the following information:
•Verifying PSTN Fallback Configuration
•Monitoring and Maintaining PSTN Fallback
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.
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