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37. NTP (Network Time Protocol)

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Why Is Time Important for Network Devices?

  • All devices (routers, switches, PCs, etc.) have an internal clock.

  • In Cisco IOS, the time can be viewed using the show clock command.

  • The show clock detail command shows the time source.

  • Over time, the internal hardware clock of a device will drift, making it unreliable as a time source.

  • From a CCNA perspective, the most important reason for accurate time is to have accurate logs for troubleshooting.

    • Example: Syslog, the protocol used for logging, relies on correct time. (Syslog will be covered in a later video).
    • Command: show logging

  • Example: Note how R3’s timestamp is completely different from R2’s.

Manual Time Configuration

  • The time on a device can be manually set with the clock set command.

  • The hardware clock (built-in clock) is the default time source, but it can be configured separately from the software clock.

Hardware Clock (Calendar) Configuration

  • Manually configure the hardware clock using the calendar set command.

  • Typically, you want to synchronize the hardware clock (calendar) with the software clock:

    • clock update-calendar — Syncs the calendar to the clock.
    • clock read-calendar — Syncs the clock to the calendar.

Configuring the Time Zone

  • The time zone can be set with the clock timezone command.

Daylight Saving Time (Summer Time)

  • Daylight saving time is set with the clock summer-time command.

  • Example:

    R1(config)# clock summer-time EDT recurring 2 Sunday March 02:00 1 Sunday November 02:00

    This command covers the start and end of daylight saving time.

Summary of Commands

NTP Basics

  • Manually configuring time on multiple devices is not scalable.

  • Over time, manually set clocks will drift, resulting in inaccurate time.

  • NTP (Network Time Protocol) allows automatic synchronization of time over a network.

  • NTP clients request the time from NTP servers.

  • A device can act as both an NTP server and an NTP client.

  • NTP can achieve time accuracy within:

    • ~1 millisecond if the NTP server is on the same LAN.
    • ~50 milliseconds if the NTP server is connected over a WAN or the internet.

  • Some NTP servers are more accurate than others. The stratum level indicates the distance from the original reference clock.

Reference Clock

  • A reference clock is a highly accurate time source, like an atomic clock or GPS clock.

  • Stratum 0: Reference clocks in the NTP hierarchy.

  • Stratum 1: NTP servers directly connected to reference clocks.

  • An NTP client can synchronize with multiple NTP servers.

NTP Configuration

  • Use the prefer keyword to prioritize a specific NTP server.

  • To display configured NTP servers:

    • sys.peer: Indicates the server that the router is currently synchronized with.
    • st: Stratum tier of the server.

  • To show NTP status:

    • Example: Stratum 2 indicates synchronization from a Stratum 1 server (e.g., Google).

  • To display NTP clock details:

  • Command to update the hardware clock with time learned via NTP:

    R1(config)# ntp update-calendar
    • The hardware clock tracks time even during power loss or device restarts.
    • When the system reboots, the hardware clock is used to initialize the software clock.

Configuring a Loopback Interface for NTP Server

  • Why configure a loopback interface for NTP on R1?
    If one of R1’s physical interfaces goes down, the NTP server remains accessible via R3’s routing path.

  • Example: Set R2’s NTP server to use R1’s loopback interface.

  • Configure R3’s NTP source servers using R1 and R2.

  • Note: R1 has a higher stratum tier, making it the preferred server over R2.

Configuring NTP Server Mode

Configuring NTP Symmetric Active Mode

  • Command to configure NTP symmetric mode:

    R2(config)#ntp peer <peer ip address>

Configuring NTP Authentication

  • NTP authentication is optional but adds security by ensuring clients only synchronize with trusted servers.

  • To configure NTP authentication:

    1. ntp authenticate — Enables NTP authentication.
    2. ntp authenticate-key <key-number> md5 <key> — Creates the authentication key.
    3. ntp trusted-key <key-number> — Specifies the trusted key.
    4. ntp server <ip-address> key <key-number> — Assigns a key to a specific server.

    Example configuration:

NTP Command Review

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