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Cisco IP Telephony Solution Reference Network Design
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Chapter 1 IP Telephony Deployment Models
Clustering Over the IP WAN
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In addition to the real-time ICCS bandwidth, intra-cluster bandwidth is required for the SQL, CTI
Manager, and LDAP traffic. The amount of additional bandwidth is dependant on the use of the
system. For instance, the use of Extension Mobility increases the amount of SQL traffic between
servers.
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A maximum Round Trip Time (RTT) of 40 ms is allowed between any two servers in the
Cisco CallManager cluster. This time equates to a 20 ms maximum one-way delay, or a transmission
distance of approximately 1860 miles (3000 km) under ideal conditions.
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The local failover model requires Cisco CallManager Release 3.1 or later.
Remote Failover Deployment Model
The remote failover deployment model provides flexibility for the placement of backup servers. Each of
the sites contains at least one primary Cisco CallManager subscriber and may or may not have a backup
subscriber. This model allows for a deployment of three to six sites with IP phones and other devices
normally registered with a maximum of four servers. (See
Figure 1-6
Remote Failover Model with Four Sites
In summary, observe the following guidelines when implementing the remote failover model:
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Configure each site to contain at least one primary Cisco CallManager subscriber and an optional
backup subscriber if desired.
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You may configure Cisco CallManager groups and device pools to allow devices to register with
servers over the WAN.
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