Clustering over the WAN

A CUCM cluster consists of one publisher server with a read/write (R/W) copy of the database. CUCM 5.0 and later uses an IBM Informix database server. The publisher database is then replicated to the subscriber servers as read/only (R/O). CUCM 6.0 added user facing features (UFF) which allows the subscriber R/O database to have limited write functionality for the following critical features to function while the publisher server is down: • Extension mobility • Call forwarding • Message waiting indicators • Device Mobility A CUCM cluster can have up to eight call processing servers, but only four of the call processing servers can be active. CUCM can have up to 20 servers in a cluster, but 12 of the servers cannot be involved in call processing. Large deployments normally have a dedicated publisher server, up to two dedicated TFTP servers, and servers dedicated to providing media resources (conferencing, media termination points, annunciators, and music on hold). The Clustering IP WAN call processing model allows companies to physically locate the publisher and subscribers at different physical locations. This model offers the highest level of availability, but has strict design limitations. Database replication between the publisher and subscriber servers have a round-trip time (RTT) delay limitation of 40ms. CUCM 6.1 has extended this limitation to 80ms, but 80ms is still a very strict requirement. The WAN must also have at least 1Mbps dedicated to database replication traffic. CUCM uses many different TCP and UDP port numbers that are documented at Cisco’s website. I have included a link to this documentation in the references section at the end of this blog. It is very important to provide communication to these ports through the firewalls or access control lists (ACL) used for security between the sites running CUCM. Due to the complexity of configuring the rules to pass this traffic, CUCM database replication traffic can be routed over an IPSEC tunnel. IPSEC traffic is normally allowed over firewalls or security devices and if it is not, only the following traffic needs to be allowed: • IP protocol 50 – encapsulation security payload (ESP) • UDP port 500 – Internet Key Association Key Management Protocol (ISAKMP) IPSEC tunnels between CUCM servers will have a performance impact. It is recommended to offload the IPSEC encryption to an appliance platform like the Cisco ASA to maintain high performance levels. REFERENCES: CUCM 7.x Solution Reference Network Design (SRND) Guide: http://www.cisco.com/en/US/netsol/ns818/networking_solutions_program_home.html www.cisco.com/go/srnd CUCM 7.0 TCP and UDP Port Usage http://tinyurl.com/cucmport