NTP

• Network Time Protocol (NTP) is a protocol to synchronize time and calendar between devices, since manually configured clocks will drift over time
  • NTP and Cisco use UTC by default, so the time zone must be set locally
  • There a certain amount of inaccuracy allowed depending on proximity to NTP Server (1millisecond for same LAN, 50ms over the WAN)
• NTP Communicates over UDP 123

NTP Modes

• Cisco devices can act in one or more three NTP modes
  • Server mode
    • Issue time to clients
  • Client mode
    • Clients request time from servers
  • Symmetric active mode
    • Two peer devices (in the same stratum) will help keep each other synchronized

NTP Stratum

• The distance between NTP Servers and the original reference clock is called the stratum
  • Reference clocks (atomic clocks or GPS clocks) are Stratum 0
    • They are the original time sources
  • Stratum 1 NTP servers are called Primary servers and get their time from the reference clocks
  • Stratum 2 from Stratum 1 And so on down the list
    • Stratum 2 and above servers are called secondary servers and they operate in both server mode and client mode
  • Anything beyond Stratum 15 is considered unreliable, and cannot be an NTP

Cisco Clocks

• Clock is a software clock, and Calendar is a hardware clock
  • Clock and Calendar are configurable in privileged exec mode
  • Default time is in UTC
• Time zone and NTP must be configured in global config mode

Commands

NTP Commands

1. config# ntp server <IP address>
   1. Adds NTP server addresses to the associations table
2. config# ntp source <interface>
   1. Configured the interface that NTP messages are programmed to send and receive from
   2. Best practice to use Loopback interfaces, since they are not depending on the status of a physical interface
3. # show ntp associations
   1. Shows all servers the NTP client is configured to connect to
   2. * indicates the server it is syncing with
   3. + indicates a server that may be synced with
   4. ~ indicates a configured (vs. dynamic?) association
4. # show ntp status
   1. Clock synchronized or not
   2. Stratum level
   3. NTP server (reference) address
5. config# ntp update-calendar
   1. Keeps the hardware Calendar up to date
6. config# ntp master <stratum level>
   1. Sets an NTP Server to act as a "Master" server on the network
   2. The associated address will be a Loopback address (NOT Loopback interface) for the local device
   3. Default stratum level is 8
      1. The loopback address (not interface) is assigned a stratum of 7
7. config# ntp peer <IP address>
   1. Configures a device as a candidate on the associations table

NTP Authentication

1. config# ntp authenticate
   1. Enable NTP authentication
2. config# ntp authentication-key <key number> md5 <key>
   1. key numbers must match between authenticating devices
   2. key is a custom password
3. config# ntp trusted-key <key number>
   1. Specifies which authentication keys from clients are trusted
4. config# ntp [server|peer] <ip address> key <key number>
   1. Identifies which key to use with which server or peer
   2. This command is only run on the client

Calendar (hardware clock)

1. # show calendar
   1. Show hardware clock config
2. # calendar set <time> <day> <month> <year>
   1. Set the hardware clock

Clock (software clock)

1. # show clock <detail>
   1. Default timezone is UTC
   2. An * at the beginning of the output indicates that the time is not considered authoritative/accurate
   3. Detail shows the source, such as hardware calendar, user config, or NTP
2. # clock set <time> <day> <month> <year>
   1. Manually set the clock time and date
3. # clock update-calendar
   1. Synchronize the hardware Calendar to the Clock time
4. # clock read-calendar
   1. Synchronize the software Clock to the hardware Calendar time
5. config# clock timezone <name for timezone> <hours offest> <minutes offset>
   1. Name does not have to match actual timezone
6. config# clock summer-time <word> <date/recurring>
   1. Word is local name for Daylight Savings time
   2. Configurations (not in Packet Tracer, maybe not important)

Metadata

OSI or TCP/IP Layer

CCNA Exam Topic

#extop-4-2

Contributors

Sources