cisco 200-301 ebook

cisco 200-301 ebook.. Broadcast: NTP periodically announces the time using a broadcast message. This method assumes that all clients are located in the local subnet.. Multicast: NTP periodically uses multicast messages to announce the time. In most cases, multicast routing must be configured to propagate the time throughout the network (multicast routing is out of the scope of this book). o. Unicast: NTP periodically (usually every 10 minutes) queries the NTP server for the current time.Most administrators implement a multicast solution because it scales best: the server sends only one message outbound, and if multicast routing is configured correctly, this message will appear in all network segments containing NTP clients. The unicast approach is typically used in smaller networks because it is easier to configure. This book describes only the NTP unicast method.Cisco IOS devices support two NTP versions: . NTPv3/v4;Chapter 17 Cisco Device Management Protocols 427. SNTP (Simple NTP) oNTPv3/v4 support MD5 authentication: a shared key is preconfigured on the time server and network device. This key is often referred to as a pre-shared key because it must exist on both devices to be used for authentication. The time server uses the MD5 and pre-shared key to calculate a hash of the time message and adds this signature to the time message. The IOS device repeats this process for the received date and time using the locally configured pre-shared key and compares the two hashes (often called the message digest or digital signature) to confirm that the message is valid.As the name implies, SNTP is a simpler method of obtaining time. Unlike NTPv3, the Cisco IOS (and most vendors) SNTP implementation does not support temporal message authentication. SNTP is not discussed in this book.You should build your own time server-often referred to as a local master clock (and not use time servers on the Internet