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 38-2. 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 38-2. NTP hierarchy in a corporate network Older versions of Windows such as Windows 2000 server do not sup- port NTP. Instead, they support 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. This is rarely a problem anymore, but watch out for old versions of any oper- ating system that doesn't play well with NTP. Chapter 38: Network Time Protocol To find publicly available Internet time servers, search the Web for "public NTP serv- ers." Some servers require registration, while others are freely available to the public. Configuring NTP NTP is a client/server application. Devices participating in NTP are either NTP servers, which provide time to other devices, or NTP clients, which request time from NTP servers. Servers are also clients and can be peered with each other as well. Configuring an IOS or NX-OS device as an NTP client is the simpler of the two models, SO we'll start there. NTP Client To configure an IOS or NX-OS device to request accurate time from an NTP server, use the ntp server command. You can list as many NTP servers as you need, each on a separate line. Here, I've listed seven publicly available NTP servers.