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cisco 200-301 exam fee.servers on the Internet because they usually do not support message authentication), and implement NTPv3o which supports MD5 authentication SNTP is a moresimple NTP, which does not support any authentication, and is therefore not recommended for use in production networks.NTP is important for the network functions of network devices, which include logging functions and authentication functions. NTP ensures that all devices are synchronized with the same time source, however, NTP does not require authentication of the time source, which allows a hacker to send NTP packets to your device and thus modify its time. Using this trick, a hacker can make it impossible for you to know exactly when the attack is taking place, or modify the time beyond the validity of the certificate so that HTTPS or IPsec authentication using the certificate fails. There are three solutions to these problems: first, use NTPv3 or use a VPN to connect to the time source; second, use (as will be discussed in Chapter 21) ACLs to filter time messages and only let messages from legitimate time sources through; and third, build your own primary time source instead of using an untrusted time source on the Internet.17.1.2 Configuring NTPIOS devices can be configured as NTP servers and/or clients. They are typically configured as time servers to relay the time from a reliable local master clock. This section only describes how to configure an NTP client on a Cisco device.Do not use the IOS device as a local master clock because the IOS does not provide a way to connect to an external reliable time source, such as GPSmethod. However, many operating systems (such as Windows and Linux) support NTP server applications thatsupport external clock connections (such as GPS) oThe following are router commands for specifying an NTP server on an IOS device.IOS(config)# ntp server IP_address [version number] [key keyid] [source interface] [refer]