Cisco IP Telephony Clock Synchronization: Best Practices
• For Cisco Unified CallManager Versions 5.0 and later:
– Manual configuration of the ntpd.conf file is not allowed.
– Cisco Unified CallManager prompts the administrator to specify the external NTP server at publisher installation and automatically populates the ntpd.conf file on the publisher server.
– The ntpd.conf files of all other cluster servers are automatically configured to point to the publisher as the NTP source.
• Configure the publisher to point to a traceable stratum 1 clock source.
• Configure network latency between the publisher and the dedicated clock source so that it is within 40 milliseconds round-trip time (RTT).
• For additional information about NTP time synchronization for Cisco Unified CallManager, please refer to the Solution Reference Network Design (SRND) guide for your specific version at http://www.cisco.com/go/srnd.
Unified CM NTP Time Synchronization Time synchronization is especially critical on Unified CM servers. In addition to ensuring that CDR records are accurate and that log files are synchronized, having an accurate time source is necessary for any future IPSec features to be enabled within the cluster and for communications with any external entity. Unified CM 5.0 and later releases automatically synchronize the NTP time of all subscribers in the cluster to the publisher. During installation, each subscriber is automatically configured to point to an NTP server running on the publisher. The publisher considers itself to be a master server and provides time for the cluster based on its internal hardware clock unless it is configured to synchronize from an external server. Cisco highly recommends configuring the publisher to point to a Stratum-1, Stratum-2, or Stratum-3 NTP server to ensure that the cluster time is synchronized with an external time source.
Otherwise, use a dedicated NTP server that synchronizes itself using GPS. The reason why I don't recommend a Cisco appliance as an authoritative NTP/SNTP server is because the clock chip is similar to a PC. It doesn't have a dedicated method of verifying. You can get a cheap NTP server that uses GPS (best method so far). Another option is to open a port in the firewall to allow the device(s) to talk to world-wide NTP pool.
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