Clustering over the IP WAN
Cisco supports CUCM clustered over an IP WAN. Figure 2-7 shows the publisher and two subscribers at one location while another pair of subscribers from the same cluster resides at a different location. The QoS-enabled IP WAN connects the two sites. Note the requirement of a round trip time less than 80 ms between the sites. This requirement is in support of database replication occurring between the publisher and all the subscribers in the cluster.
Figure 2-7 Clustering over the WAN
Some of the characteristics of this model include:
Applications and CUCM servers of the same cluster can be distributed over the IP WAN.
The IP WAN carries intracluster server communication and signaling.
Limited number of sites:
Two to four sites for local failover (two CUCM servers per site)
Up to eight sites for remote failover across the IP WAN (one CUCM server per site).
The cluster design is useful for customers who require more functionality than the limited feature set that is offered by Cisco Unified SRST. This network design also allows remote offices to support more IP phones than SRST if the connection to the primary CUCM is lost.
Design Guidelines for Clustering over WAN Deployment Model
Although the distributed single-cluster call-processing model offers some significant advantages, it must adhere to these strict design guidelines:
Two CUCM servers in a cluster must have a maximum round-trip delay of 80 ms between them. Because of this strict guideline, this design can be used only between closely connected, high-speed locations.
A minimum of 1.544 Mbps (T1) of bandwidth is required for Intra-Cluster Communication Signaling (ICCS) between each site and every other site that is clustered over the WAN. This bandwidth supports up to 10,000 busy hour call attempts (BHCAs) within the cluster. The BHCA represents the number of call attempts that are made during the busiest hour of the day.
In addition to the bandwidth required for ICCS traffic, a minimum of 1.544 Mbps (T1) of bandwidth is required for database and other inter-server traffic between the publisher and every subscriber node within the cluster.
Up to eight small sites are supported using the remote failover deployment model. Remote failover allows you to deploy one server per location. (A maximum of eight call-processing servers are supported in a cluster.) If CUCM fails, IP phones register to another server over the WAN. Therefore, Cisco Unified SRST is not required in this deployment model (although it is supported). The remote failover design may require significant additional bandwidth, depending on the number of telephones at each location.
Benefits of Clustering over WAN Deployment Model
Clustering over the IP WAN provides a combination of the benefits of the two multisite deployment models to satisfy specific site requirements.
Although there are stringent requirements, clustering over the IP WAN offers these advantages:
Single point of administration for users for all sites within the cluster
Shared line appearances
Cisco Extension Mobility within the cluster
A unified dial plan
The clustering over IP WAN design is useful for customers who want to combine these advantages with the benefits that are provided by a local call-processing agent at each site (intrasite signaling is kept local, independent of WAN failures) and require more functionality at the remote sites than is provided by Cisco Unified SRST. This network design also allows remote offices to support more Cisco IP phones than SRST (1500 IP phones using Cisco 3945E Integrated Services Routers) in the event of WAN failure.
These features make clustering across the IP WAN ideal as a disaster-recovery plan for business continuance sites or as a single solution for up to eight small or medium sites.