Inter-Gateway Alternate Routing

Last Updated : Sep 29, 2026 |
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With Inter-Gateway Alternate Routing (IGAR), Communication Manager can use the PSTN instead of the IP-WAN for bearer connections. This feature is beneficial when the IP-WAN cannot carry the bearer connection for the single-server systems that use the IP-WAN to connect bearer traffic between port networks or gateways.
Note:
Communication Manager Release 6.3.5 and earlier supported IGAR for analog, DCP, and H.323 endpoints. Communication Manager Release 6.3.6 extends this support to SIP endpoints.
IGAR requests PSTN to provide bearer connections in any of the following conditions:
  • Reaching the number of calls or bandwidth allocated through Call Admission Control- Bandwidth Limits (CAC-BL).
  • Facing VoIP RTP resource exhaustion in a port network or media gateway.
  • Encountering the codec set between a pair of network regions set to pstn.
  • Finding forced redirection configured between a pair of network regions.
IGAR provides enhanced Quality of Service (QoS) to large, distributed single-server configurations. IGAR is intended for configurations where the IP network is not reliable enough to carry bearer traffic. If you have more than one IP network available, you can use H.323 or SIP trunks for IGAR instead of the PSTN.
When Communication Manager needs an inter-gateway connection and adequate IP bandwidth is unavailable, Communication Manager attempts to substitute a trunk connection for the IP connection. For example, Communication Manager can substitute a trunk connection in any of the following situations:
  • A user in one Network Region (NR) calls a user in another NR
  • A station in one NR bridges on to a call appearance of a station in another NR
  • An incoming trunk in one NR routes to a hunt group with agents in another NR
  • An announcement or music source from one NR must be played to a party in another NR
Communication Manager attempts to use a trunk for inter-region voice bearer connection when the following five conditions are met:
  • An inter-gateway connection is needed.
  • IGAR requests PSTN to provide bearer connections.
  • IGAR is enabled for the NRs associated with each end of the call.
  • The Enable Inter-Gateway Alternate Routing system parameter is set to y.
  • The number of trunks, used by IGAR in each NR, has not reached the limit administered for that NR.
The SRC PORT TO DEST PORT TALKPATH page of the status station screen shows the IGAR trunk connectivity for an inter-NR call.
A Trunk Inter-Gateway Connection (IGC) is established using ARS to route a trunk call from one NR to IGAR Listed Directory Number (LDN) extension administered for another NR. The Trunk IGC is independent of the call. Therefore, Communication Manager can originate the IGC from the NR of the calling party to the NR of the called party, or vice versa. Some users use Facility Restriction Levels or Toll Restriction to determine who gets access to IGAR resources during a WAN outage. For these users, the calling user is considered the originator of the Trunk IGC for authorization and routing. For outgoing trunk groups administered to send the Calling Number, the IGAR Extension in the originating NR is used to create this number using the appropriate administration.
A few examples of failure scenarios and how Communication Manager handles the scenarios:
  • On a direct call, the call continues to the first coverage point of the unreachable called endpoint. If no coverage path is assigned, the calling party hears a busy tone.
  • If the unreachable endpoint is accessed through a coverage path, the coverage point is skipped.
  • If the unreachable endpoint is the next available agent in a hunt group, that agent is considered unavailable. The system tries to route the call to another agent using the administered group type, such as Circular distribution and Percent Allocation Distribution.