Call Flow with CallBridge Group On Cisco Meeting Server Demystified


The concept of Call Bridge Group and the importance of “accept replaces header” in the SIP Trunk Security Profile and The “Rerouting Calling Search Space” on CUCM in the SIP Trunk that points to CMS

The primary goal of the Call Bridge Group feature is to make sure that all participants within a Call Bridge Group accessing a particular Space will all be connected to the same server, thereby eliminating excessive distribution of calls between Call Bridge nodes. To make this happen, when a call is extended to a CMS to join a Space, the server will communicate with the other Call Bridge servers to determine which server is currently hosting the meeting. If it determines that a Space is being hosted by another server within the Group, then the destination serer will send a SIP INVITE with a Replaces header, essentially telling the Unified CM to transfer the call to it.

At the same time, if a meeting is hosted by a Call Bridge in one Call Bridge Group and someone wants to join from a different region and therefore dials in via a second Call Bridge Group, those calls will always be distributed between the servers in the different regions. The goal is to aggregate the calls for a Space on a single server within a given Call Bridge Group.

This feature imposes requirements on the call control. For Expressway, this means version X8.11 or later and the Meeting Server Load balancing option configured on Zones pointing to CMS, which is the default for all Zones using the Custom Zone profile.

For a CMS integration to Unified CM, this means Accept replaces header must be enabled in the SIP Trunk Security Profiles and each SIP trunk to CMS must have a Rerouting Calling Search Space configured to allow the call be be transferred between CMS SIP trunks.


Call Bridge clustering allows multiple Call Bridges to operate as a single entity and scale beyond the capacity of any single Call Bridge.

Call routing without Call Bridge Group

  1. User-1 Dials ccnp@collab.lab.local
  2. CUCM checks SIP Route Pattern
  3. CUCM checks Route List RL-CMS
  4. CUCM checks Route Group RG-CMS
  5. Route Group RG-CMS is configured with 3 SIP Trunks to the 3 CMS CMS-1, CMS-2 and CMS-3 in Circular Distribution Algorithm
  6. CUCM Routes call to CMS-1
  7. User-2 Dials ccnp@collab.lab.local
  8. CUCM Routes call to CMS-2
  9. User-3 Dials ccnp@collab.lab.local
  10. CUCM Routes call to CMS-3

What’s the ISSUE?

3 ports used on CMS-1, CMS-2 and CMS-3.

Total of 9 ports used for 3 party conference.

Disadvantages of Call Bridge Clustering: Inefficient port utilization

Call routing with Call Bridge Groups and Intelligent Load balancing

Introduced in CMS 2.1
To group Call Bridges in one Group. Enables Load Balancing of Calls on CallBridge within same CallBridge Groups.
Within each Call Bridge group, the aim is to have calls for the same conference placed on the same server whenever possible.

To do that Call Bridge Groups require Cisco Call Manager SIP Trunk Security Profie Requires “ACCEPT with Replaces” to accept “INVITE with Replaces” header.

  1. User-1 Dials ccnp@collab.lab.local
  2. CUCM checks SIP Route Pattern
  3. CUCM checks Route List RL-CMS
  4. CUCM checks Route Group RG-CMS
  5. Route Group RG-CMS is configured with 3 SIP Trunks to the 3 CMS CMS-1, CMS-2 and CMS-3 in Circular Distribution Algorithm
  6. CUCM Routes call to CMS-1
  1. User-2 Dials ccnp@collab.lab.local
  2. CUCM Routes call to CMS-2
  3. CMS-2 sends a SIP INVITE with replaces to CUCM to re-route the call to CMS-1
  4. CUCM routes call to CMS-1
  1. User-3 Dials ccnp@collab.lab.local
  2. CUCM Routes call to CMS-3
  3. CMS-3 sends a SIP INVITE with replaces to CUCM to re-route the call to CMS-1
  4. CUCM routes call to CMS-1

Benefits: Total of only 3 ports used for 3 party conference rather than 9

Categories: Collaboration

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