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InterCluster Trunk (ICT) issue

Hi Cisco,

Good day!

We have an issue on our ICT configuration. We experienced fast busy tone after the CUCM failover.

Here is the scenario:

CUCM-ClusterA --> ICT --> CUCM-ClusterB

1) We conduct test call from IP phone registered in clusterA to IP phone ClusterB. 

2) While the call is ongoing we have tried to shutdown the Publisher then failover to subscriber. The call was not dropped.

3) We have tried another call from ClusterA to ClusterB and it gives us fast busy tone only.

4) After another hours, we have tried to call again and it successful.


May we know what is the cause of the issue why the ICT not working after failover?


Thanks and regards,



have you configured Cluster A

have you configured Cluster A's pub & sub IP address in Cluster B?

Is it SIP trunk or non-GK ICT? what is the CUCM version?

//Suresh Please rate all the useful posts.

Hi Suresh, Thanks for your

Hi Suresh,


Thanks for your immediate reply.

What do you mean? if we configured Cluster A's Pub and Sub IP address on the ICT?

We configured non-GK ICT, and Cluster A has ver. 8.6 and Cluster B ver. 10.0.

Hi Ralph, When you trying to

Hi Ralph,


When you trying to call from Cluster A to Cluster B with publisher shut down in cluster A, then the SIP Invite message are sent with SUB IP address and this same SUB address must be present in the SRV destination IP address of Cluster B Trunk configuration.

Since it's provide a security feature where the receiving cluster won't accept any call if the sending IP adddress is not present in its configuration.

You must be getting error 503 service not available when you dialling from Cluster A.

This error send by CUCM (or  gateway) only when the IP address is not recognized by the receiving CUCM.

so please put the all IP address of the cluster (pub & sub) in the trunk configuration.

If it was SIP Trunk then it would had given you RUN ON ALL MODE ACTIVE option to enable it.

SIP Trunk is much better compared to ICT Trunk.




Let me clarify the scenario

Let me clarify the scenario:

We have already defined all IP address of CUCM from Cluster B on the ICT registered from Cluster A. When we shutdown one CUCM from Cluster B; failover, then try another call we are getting fast busy tone.

And we have also observed that if we dial other route pattern that going to ICT, the call was successful.

Please share the SDL/SDI logs

Please share the SDL/SDI logs to get more understanding of this.

When you say dial route pattern that going to's confusing friend.

Please explain.

Also In Cluster A : ICT Trunk should have the IP address of the Cluster B CUCM in IP address field.

In Cluster B: ICT Trunk should have the IP address of the cluster A CUCM in Ip address field.






Hi Ralph,Did you get any

Hi Ralph,

Did you get any RouteList Exhausted alert from RTMT for this call failure? As Raaj said, we need to look into the CCM traces for this issue.

Also There is a known issue on the non GK ICT configured on the versiom 8.X & 9.X

The best practice is to configure SIP Trunk between CUCM Clusters if the version is 8.X and above.



Please rate all the helpful posts.

//Suresh Please rate all the useful posts.

Here is the Snippet from CUCM

Here is the Snippet from CUCM 8.x SRND about SIP Trunk preference.

General Deployment Considerations

Unified CM SIP trunks offer a greater set of features in comparison with H.323 intercluster trunks, thus making SIP the protocol of choice for intercluster trunk connections (although H.323 Annex M1 may still be preferred for intercluster trunk connections to Unified CM clusters using earlier software versions). Also, given the wide support of SIP in the industry, SIP trunks are usually a good choice for connectivity to third-party applications and service providers.



Snippet from CUCM 10.x SRND

While H.323 trunks are still supported, SIP trunks are the recommended trunk type for Unified Communication deployments because SIP trunks provide additional features and functionality not available with H.323 trunks.




Please rate all the helpful posts

//Suresh Please rate all the useful posts.
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