Cisco Unified Communications Manager (CUCM).
The following information illustrates the two basic approaches for time synchronization when installing Cisco Unified Communications Manager (CUCM) using VMware :
-The Network Time Protocol (NTP, RFC 1305) is a simple protocol using UDP port 123. The RFC describes the protocol itself as well as the architectural framework and in-depth implementation recommendations.
-The NTP architectural framework specifies a hierarchy of time servers using stratum values from one to sixteen to indicate their relative accuracy.
-The most accurate servers using external clocks (GPS receivers are commonly used due to their low cost) are stratum one servers and any server synchronizing itself to a stratum X server advertises itself as stratum X+1 server.
-Internal NTP Server:
-you can configure an internal NTP server to synchronize time with,if so this internal NTP server could be a Windows machine,Unix machine,Cisco IOS gateway that will be configured as an internal NTP server and when configuring this in a lab environment you will need to use GNS3 and any IOS image that supports the NTP daemon.Use the " ntp master x " where is x is the NTP master stratum number,it's NOT recommended to be higher than 6 so the lower is better also you can use the " show ntp status " command to verify your configuration.
Notice:If you decided to use an internal NTP server for time synchronization there's NO need to configure the CUCM virtual network adapter to Bridged or NAT since you do NOT need any internet connectivity to reach your internal NTP server so usually you will configure it as " Internal " also you must configure the virtual network adapter that will be created on your physical host on the same subnet and to use the same gateway IP address your CUCM is using.
-External NTP Server:
-Another option you can use " External NTP Server ".One of the advantage when using external NTP server instead of internal NTP server is that you do NOT need to use GNS3 and emulate NTP server on a Cisco IOS image anymore.All you need is an internet connectivity to the public NTP pool.
-One of the most popular public NTP pool projects is pool.ntp.org that has more than (3611) public NTP server around the world in the Global Zone.It's recommended to choose the nearest NTP pool to your country and you will see the available NTP servers list as follows depending on your choice.
-Choose the NTP pool you want to synchronize time with and ping it and now you can use this public IP address as your external NTP server when installing CUCM or any of the Cisco Unified Communications Solutions.
Notice:Since you will synchronize the CUCM time with an external NTP server you must configure the CUCM virtual network adapter to Bridged or NAT to enable your CUCM to reach the NTP server.