One day during an integration of a Cluster of Cisco CUCM with a Cluster of Cisco Meeting Server, I was asked why do we need to enable the Run On All Active Unified Node option on both Route List and SIP Trunk.
After in depth analysis with wireshark, I wrote an explanation with a chart call flow.
To first understand this option, first you need to understand that this option affects the outgoing call, more precisely when an endpoint let's say the HQ-Phone-1 registered to HQ-PUB dials the number of a space, and the call SIP INVITE is sent to the call control where the phone is registered to (the "From" field and the Host Part of the URI in the SIP INVITE Header). The question that arises is : once the call is received by the Call Control, who is the initiator of the outgoing call in the Cluster CUCM with multiple Nodes running the Call Manager Services?
The answer it depends, below you can see in some scenarios, the calling device is the Route List, NOT the endpoint, let me explain it :
When the Route Pattern points to the Route List. In this case the calling device is the Route List.
· If the Route List is not registered to node which is a member of the CUCM group assigned to the SIP Trunk, then the node will randomly distribute the calls accross the nodes in the Trunk’s CUCM group - the max is 3 servers.
· If the Route List is registered to a node which is also a member of the CUCM Group assigned to the SIP Trunk, then the node on which the Route List is registered is used to initiate the outbound call.
To summarize, the following definitions should be taken into consideration:
1-Run on All Active Unified CM Nodes disabled on both Route List and SIP Trunk. The calling device is the Route List.
2-Run on All Active Unified CM Nodes enabled on Route List, disabled on SIP Trunk. The calling device is the Endpoint.
3-Run on All Active Unified CM Nodes disabled on Route List, enabled on SIP Trunk. The calling device is Route List .
4-Run on All Active Unified CM Nodes enabled on both Route List and SIP Trunk. The calling device is the Endpoint.
As you can notice, The Route List is only active on one CUCM server, In this case, if the Run On All Active Unified CM Node is NOT checked on both Route List and SIP Trunk, all outgoing calls can be initiated by the same node where the route list is registered ( in other words as mentioned previously when the Route List is registered to a node which is also a member of the CUCM Group assigned to the SIP Trunk). And this is not recommended.
Solution: enable the "option Run on All Active Node" on both Route List and SIP Trunk, it gives you a greater amount of nodes to use for outgoing call routing. When this option is enabled, the UCM Group for both Route List and SIP Trunk is IRRELEVANT......
Regards