Hi,
Subscriber is installed by pointing it to the Publisher Ip during the time of installation. The subscriber can be used as a backup at various places on cucm like the following:
1) Callmanager group ( to allow phones to register with backup cucm in case top one is not available
2) By configuring services with separate URL's containing Pub or Sub IP for redundancy
3) SIP trunks and ICT's with multiple destination servers for backup / load balancing
4) Option 150 for TFTP with two ip addresses ( Pub and Sub ) to allow the backup to take over in case primary is down
These are just a few that strike my mind right now. You should read the CUCM SRND for any version like 10.x or 11.0 and check the call processing and deployment models sections to understand this Pub / Sub relationship in detail. the active-active or active-standby is not the correct term for CUCM Pub and Sub, it is much broader than being simply an active-active or active-standby concept.
http://www.cisco.com/c/en/us/td/docs/voice_ip_comm/cucm/srnd/collab10/collab10/callpros.html#pgfId-1148520
Manish
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