Before your network becomes operational, you can take several proactive steps to make troubleshooting easier, including:
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Produce network topology diagrams to help you isolate potential sources of problems.
•Synchronize the date and time on all servers.
Network Topology Diagrams
One of the first lines of defense is possessing current topology information. One of the most important pieces of topology information is a detailed network diagram (usually created using Microsoft Visio or a similar application). At a minimum, your network topology diagrams should include the following information:
•The name assigned to each major device (typically the DNS name)
•IP addresses for all devices in the network
–Addresses for each router, core and access switch
–Addresses for all telephony and application servers, including the IP address for each server in a Cisco Unified Communications Manager cluster
–DHCP address range for addresses assigned to endpoints such as IP phones and agent workstations
•Phone extension number ranges assigned to sets of agents or users, as well as the main inbound dial-up numbers for each location. This information is useful in resolving dial plan configuration errors.
•WAN IP and PSTN links between sites.
This information is critical for isolating which components are involved in a particular problem. For medium- to large-sized networks, you may want to take a "layered" approach in your diagrams. Create a high-level diagram that illustrates the overall physical layout of your network, including all sites and the links between them. Then for each site create additional diagrams that show detailed addressing information, port numbers and dial plan configurations.
Tip Frequent adds, changes and upgrades to your network can quickly make these diagrams out-of-date. Inaccurate diagrams slow down the troubleshooting process and may lead to misdiagnosing the problem. Remember to keep these diagrams as current as possible.
Synchronizing Server Date and Time
The best resources for diagnosing problems within your network are the debug and trace log files produced by individual Cisco devices. Tracing can be enabled on multiple devices and the log file output compared to isolate problems. In order to correlate messages for the same activity in different log files, you must compare the message timestamps and the source device MAC and IP addresses (there is no universal call ID value shared between Cisco devices). You should synchronize every device to the same date and time source so that the timestamps match. To accomplish this synchronization, set each device to obtain its date and time from the same Network Time Protocol (NTP) source.
For Cisco IOS-based devices (switches, routers or voice gateways), you can configure each device to act as a NTP client and periodically poll a master NTP source using the following command:
ntp server ip-address [version number] [key keyid] [source interface] [prefer]
Additional IOS commands are available to establish a device as a NTP peer (operating as the master source for other devices), as well as setting up NTP broadcasting instead of polling. See the Cisco IOS Configuration Fundamentals Command Reference 
for details on these IOS commands.