e last save's date was in the future. The company ended up losing an entire day's worth of trans- actions because of my inability to correctly deploy NTP. Chapter 35: Network Time Protocol The problem boiled down to my belief that I only needed to configure a single NTP server. I assumed that all time would be accurate from that point on because I had configured the network devices to learn accurate time from a third party. When designing NTP for a network, I still like to limit the number of devices that will make inquiries from the Internet. Usually, I make the two core switches the NTP servers for the entire network. I configure these two switches (assuming a high- availability pair) to receive time from a minimum of three Internet time servers. The switches also act as NTP peers. An NTP peer is considered to be an equal in a stratum. This allows each switch to learn time from the other as an additional source. I then configure all other networking devices to receive their time from the core switches. Assuming the Internet time servers are all operating at stratum one, the core switches should operate at stratum two. The rest of the switches in the network operate at stratum three. This hierarchy is shown in Figure 35-2. Atomic clock #1 Atomic clock #2 Atomic clock #3 Atomic clock #4 Stratum 0 accurate Stratum 1 Peers Stratum 2 accurate Stratum 3 Least accurate Figure 35-2. NTP hierarchy in a corporate network Older versions of Windows do not support NTP. Instead, they sup- port Simple NTP (SNTP). Because of this limitation, you cannot use Cisco devices as NTP servers for these older Windows machines without adding a true NTP client to them. To find publicly available Internet time servers, search the Web for "public NTP servers." Some servers require registration, while others are freely available to the public. NTP Design Configuring NTP NTP is a client/server application. Devices participating in NTP are either NTP serv- ers, which provide time to other devices, or NTP clients, which request time from NTP s