I am trying to undertand the function of CMG field when we create a Route List i.e what impact does the RL CMG have on the call path.
VM Route Pattern: 408-123-4567
CUC_RL with CMG_Default
CUC_RG which contains following two SIP Trunks
Trunk_A (with DP_A which contains CMG_DC01)
Trunk_B (with DP_B which contians CMG_DC02)
My understanding is that with this configuration, under normal situation, call path will be via the primary UCM in CMG_DC01.
In case Trunk_A is down, the path will be via Trunk_B which will use the primary UCM in CMG_DC02.
Am I correct? And if so, what role did the CMG in RL play?
There are some other considerations here before the exact routing
of the call path can be determined Maybe this will help;
Multiple Source Unified CM Servers for Originating SIP Trunk Calls
Using Standard Unified CM Groups
The nodes defined in the Unified CM Group associated with an individual trunk make up the set of servers that can place or receive calls over that trunk. Up to three nodes can be defined in a Unified CM Group, thus ensuring high availability of the trunk itself.
Using Run on All Active Unified CM Nodes
The Run on all Active Unified CM Nodes feature creates and enables a SIP trunk instance on each call processing subscriber within the cluster, thus allowing these nodes to place or receive calls over the trunk.
The Unified CM Route Local Feature And Its Effect on Subscriber Selection for Outbound SIP Trunk Calls
The Route Local feature in Unified CM is designed to reduce intra-cluster traffic. The feature operates as illustrated by the following example:
When a device such as a phone is making an outbound call over SIP Trunk 1, if an instance of SIP Trunk 1 is active on the same node as the one to which the phone is registered, then always use this co-located SIP Trunk 1 instance rather than internally routing the call to another SIP Trunk 1 instance on another node within the cluster.
The effect of the Route Local feature on node selection depends on whether Unified CM Groups or Run on all Active Unified CM Nodes is configured on the trunk. For trunks with Run on all Active Unified CM Nodes configured, the node to which the calling device is registered is used to make the outbound SIP trunk call. When Unified CM Groups are used on the trunk, if the calling device is registered to one of the nodes in the trunk's Unified CM Group, then the Route Local rule applies. If the calling device is not registered to one of the nodes in the trunk's Unified CM Group, then Unified CM will randomly distribute the call over the nodes in the trunk's Unified CM Group.
Using Run on all Active Unified CM Nodes is the recommended approach for SIP trunks because it allows call distribution across nodes to be determined by the calling device and it minimizes intra-cluster traffic.
Multiple Destination IP Addresses per SIP Trunk
A single SIP trunk can be configured with up to 16 destination IP addresses. Unified CM uses random distribution to the configured destination IP addresses when placing calls over a SIP trunk. Using multiple IP addresses on a SIP trunk can help to reduce the need to deploy multiple trunks with route lists and route groups.
Design Considerations When Using Run on All Active Unified CM Nodes
When using Run on All Active Unified CM Nodes in conjunction with multiple destination addresses, be aware that to accept inbound calls, the inbound source IP address received on the SIP trunk must match with a configured destination IP address on the inbound trunk. For example, if Run on all Active Unified CM Nodes is configured on the SIP intercluster trunk in each cluster, then each trunk must be configured with the corresponding destination address of every active node in the destination cluster. Where clustering over the WAN designs are deployed and geographic call distribution and failover are required, use standard Unified CM Groups on multiple intercluster trunks (each with up to three destination IP addresses) in conjunction with route lists and route groups.
Multiple SIP Trunks Using Route Lists and Route Groups
Multiple prioritized SIP trunks are often required to address failure scenarios in Unified Communications designs. These trunks should be configured in route groups in a single route list and associated with a route pattern. If Unified CM is not able to place a call over the selected trunk in the list, it will try the next trunk in the list. As a general recommendation, enable Run on all Active Unified CM Nodes for all route lists.
From the 8.x SRND
"Always movin' ahead and never lookin' back" - Springsteen