Cisco Unified Communications Manager automatically reroutes calls
through the PSTN or other networks when Cisco Unified Communications Manager
blocks a call due to insufficient location bandwidth. With automated alternate
routing, the caller does not need to hang up and redial the called party. The
Automated Alternate Routing (AAR) feature enables Cisco Unified Communications
Manager to establish an alternate path for the voice media when the preferred
path between two intra-cluster endpoints runs out of available bandwidth, as
determined by the locations mechanism for CAC. This document provides a sample
configuration for Automated Alternate Routing.
Ensure that you meet these requirements before you attempt this
Here in this scenario you already have CAC implemented after the
configuration of locations and deployment of RSVP agents to prevent WAN
bandwidth oversubscription. You implement AAR to route calls over the PSTN if
they were not admitted by locations-based CAC. The AAR feature applies
primarily to Centralized Call Processing deployments.
Note: Use of globalized call routing simplifies AAR implementation in
The information in this document is based on these software and
The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.
Refer to the Cisco Technical Tips
Conventions for more information on document conventions.
When a call is made from the device of one location to the device of
another location, location bandwidth gets deducted from the maximum available
bandwidth that is available for the call at either location. If not enough
location bandwidth for the call exists at either location, instead of blocking
the call, Cisco Unified Communications Manager uses the table of AAR groups and
the external number of the terminating directory number to supply the alternate
number that is used to reroute the call through the PSTN or other network. The
Cisco Unified IP Phone displays the Network congestion,
rerouting. message. Use Service Parameters Configuration for
the Cisco CallManager service in order to configure this message.
AAR supports these call scenarios for insufficient bandwidth:
Call originates from a line or directory number (DN) of an IP phone
within one location and terminates to a line or DN of another IP phone within
another location. This scenario includes calls that terminate at the shared
line with terminating IP phone devices that are resident in multiple locations
and calls that terminate at the Cisco voice-mail port.
Incoming call through a gateway device within one location terminates
to a line or DN of an IP phone within another location. This scenario includes
calls that terminate at the shared line with terminating IP phone devices that
are resident in multiple locations and calls that terminate at the Cisco
Cisco Unified Communications Manager automatically attempts to reroute
calls, due to insufficient bandwidth, through the PSTN or other network only
when the AAREnable enterprise parameter is set to true. Cisco Unified
Communications Manager uses the device-based AAR calling search space, which is
assigned to Cisco Unified IP Phone station devices and gateway devices, when it
attempts to route the call to the gateway device that connects to the PSTN or
other network. Cisco Unified Communications Manager uses the external phone
number mask and the directory number of the line or DN and the Cisco voice-mail
port to derive the alternate number that is used to reroute the call.
For example, if a phone in the headquarters calls a phone in branch A
and the available bandwidth for the WAN link between the branches is
insufficient (as computed by the locations mechanism), AAR can reroute the call
through the PSTN. The audio path of the call is IP-based from the calling phone
to its local PSTN gateway, TDM-based from that gateway through the PSTN to the
branch A gateway, and IP-based from the branch A gateway to the destination IP
The implementation of AAR includes these steps.
Configure partitions and CSSs.
Configure AAR groups.
Configure phones for AAR:
ApplyAARCSS and (source) AARgroup to IP phones.
Configure IPphone directory numbers:
Apply (destination) AAR group.
Set individual AAR destination mask (CFNB)
Forward to voice mail if no bandwidth is
Note: Use the
Command Lookup Tool
(registered customers only)
to obtain more information on
the commands used in this section.
This document uses this network setup:
In these steps, you set the Cisco CallManager service parameter
Automated Alternate Routing Enabled to True in order to enable
Choose System > Service Parameters and choose
the Cisco Unified Communications Manager.
From the Service drop-down menu, choose Cisco
Locate the Clusterwide Parameters (System—CCM Automated Alternate
Set the Automated Alternate Routing Enable parameters
Complete these steps:
Choose Call Routing>AAR groups and click
Enter the name hq_aar.
Note: These same previous steps have to be performed in order to create
another AAR group called br_aar for Branch office also.
Complete these steps:
Choose Device > Phone and click
Choose any phone which is in the HQ partition.
Click Line 1 in order to get to the Directory
Number Configuration window.
In the AAR Settings pane, choose hq_aar for the
AAR Group. For branch office phones, this has to be
Verify that the external phone number mask is in globalized
From the Related Links, choose Configure Device
and click Go.
At the Phone Configuration window, choose
Global_css for the AAR CSS.
Click Save and then OK in the
Reset the phone.