Cisco: Very simple NTP configuration

NTP (Network Time Protocol) is usually very simple to configure on Cisco devices. Of course you can reach complex configuration, but since I work in this field I didn’t saw somebody to push the things to extreme in NTP configuration.

NTP is based on server – client relation. It is recommended that in a Cisco network environment, you should use online the client part of the NTP, and to choose some external NTP sever to synchronize with.  This is because using a NTP server (master) on your networ  violates NTP’s hierarchical trust model. You should use a NTP master only is there is no possibility to reach an external one or if some corporate policies dictate this.

NTP can operate in 4 different modes. See a short explanation below:
-> Client:  A NTP client is configured to let its clock be set and synchronized by an external NTP timeserver. NTP clients can be configured to use multiple servers to set their local time and are able to give preference to the most accurate time sources. They will not, however, provide synchronization services to any other devices.
-> Server: A NTP server is configured to synchronize NTP clients. Servers can be configured to synchronize any client or only specific clients. NTP servers, however, will accept no synchronization information from their clients and therefore will not let clients update or affect the server’s time settings.
-> Peer: With NTP peers, one NTP-enabled device does not have authority over the other. With the peering model, each device shares its time information with the other, and each device can also provide time synchronization to the other.
-> Broadcast / Multicast: Broadcast/multicast mode is a special server mode with which the NTP server broadcasts its synchronization information to all clients. Broadcast mode requires that clients be on the same subnet as the server, and multicast mode requires that clients and servers have multicast access available and configured.

For our simple NTP configuration we will use the server and  client mode. For this tutorial we will use the same topology like in the post “Cisco: BGP path selection for outgoing traffic” where we have already a working BGP environment. If you do not have the topology, you can download it here. Please see the tutorial below: