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1Networking Devices2Interfaces And Cables3OSI Model and TCP/IP Suite4Intro to the CLI5Ethernet LAN Switching - Part 16Ethernet LAN Switching - Part 27IPv4 Addressing - Part 18IPv4 Addressing - Part 29Switch Interfaces10The IPv4 Header11Routing Fundamentals - Part 111Static Routing - Part 212Life of a Packet13Subnetting - Part 114Subnetting - Part 215Subnetting (VLSM) - Part 316VLANs - Part 117VLANs - Part 218VLANs - Part 319DTP / VTP20Spanning Tree Protocol - Part 121Spanning Tree Protocol - Part 222Rapid Spanning Tree Protocol23Etherchannel24Dynamic Routing25RIP / EIGRP26OSPF - Part 127OSPF - Part 228OSPF - Part 329First Hop Redundancy Protocol30TCP and UDP31IPv6 - Part 132IPv6 - Part 233IPv6 - Part 334Standard Access Control List35Extended Access Control List36CDP and LLDP37NTP - Network Time Protocol38DNS - Domain Name System39DHCP - Dynamic Host Configuration Protocol40SNMP - Simple Network Management Protocol41SYSLOG42SSH - Secure Shell43FTP and TFTP44NAT (Static) - Part 145NAT (Dynamic) - Part 246QoS (Voice VLAN) - Part 147QoS (Quality of Service) - Part 248Security Fundamentals49Port Security50DHCP Snooping51Dynamic ARP Inspection52LAN Architectures53WAN Architectures54Virtualization and Cloud - Part 154Virtualization (Containers) - Part 254Virtualization (VRF) - Part 355Wireless Fundamentals56Wireless Architectures57Wireless Security58Wireless Configuration59Introduction to Network Automation60JSON, XML, and YAML61REST APIs62Software Defined Networking63Ansible, Puppet, and Chef67CCNA Complete Cheatsheet
/NTP - Network Time Protocol
Topic 374 min read

NTP - Network Time Protocol

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37. NTP

WHY IS TIME IMPORTANT FOR NETWORK DEVICES?

  • All DEVICES have an INTERNAL CLOCK (ROUTERS, SWITCHES, PCs, etc)
  • In CISCO IOS, you can view the time with the show clock command

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  • If you use the show clock detail command, you can see the TIME SOURCE

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  • The INTERNAL HARDWARE CLOCK of a DEVICE will “drift’ over time, so it’s NOT the ideal time source.

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

  • Syslog, the protocol used to keep device logs, will be covered in a later video

Command: show logging

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Note : R3’s time stamp is completely different than R2’s !!!

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MANUAL TIME CONFIGURATION

  • You can manually configure the TIME on the DEVICE with the clock set command

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  • Although the HARDWARE CALENDAR (built-in clock) is the DEFAULT time-source, the HARDWARE CLOCK and SOFTWARE CLOCK are separate and can be configured separately.

HARDWARE CLOCK (CALENDAR) CONFIGURATION

  • You can MANUALLY configure the HARDWARE CLOCK with the calendar set command

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  • Typically, you will want to SYNCHRONIZE the ‘clock’ and ‘calendar’
  • Use the command clock update-calendar to sync the calendar to the clock’s time
  • Use the command clock read-calendar to sync the clock to the calendar’s time

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CONFIGURING THE TIME ZONE

  • You can configure the time zone with the clock timezone command

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DAYLIGHT SAVING TIME (SUMMER TIME)

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Full command :

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

This covers the START of Daylight Savings and the end of Daylight Savings

SUMMARY OF COMMANDS

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NTP BASICS

  • Manually configuring the time on DEVICES is NOT Scalable
  • The manually configured 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 be an NTP SERVER and an NTP CLIENT at the same time
  • NTP allows accuracy of TIME with ~1 millisecond if the NTP SERVER is in the same LAN - OR within ~50 milliseconds if connecting to the NTP SERVER over a WAN / the INTERNET
  • Some NTP SERVERS are ‘better’ than others. The ‘distance’ of an NTP SERVER from the original reference clock is called stratum
<aside> 💡 NTP uses UDP port 123 to communicate </aside>

REFERENCE CLOCK

  • A REFERENCE CLOCK is usually a VERY accurate time device like an ATOMIC CLOCK or GPS CLOCK
  • REFERENCE CLOCKS are stratum 0 within the NTP hierarchy
  • NTP SERVERS directly connected to REFERENCE CLOCKS are stratum 1

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(Peering with Devices is called …)

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  • An NTP CLIENT can SYNC to MULTIPLE NTP SERVERS

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NTP CONFIGURATION

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Using key argument “prefer” makes a given server the PREFERRED SERVER

(To show configuration servers)

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sys.peer = This is the SERVER that the current ROUTER (R1) is being synchronized to

st = Stratum Tier

(To show NTP Status)

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stratum 2 because it’s synchronizing from Google (stratum 1)

(To show NTP clock details)

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This command configures the ROUTER to update the HARDWARE CLOCK (Calendar) with the time learned via NTP

R1(config)# ntp update-calendar

The HARDWARE CLOCK tracks the DATE and TIME on the DEVICE - even if it restarts, power is lost, etc.

When the SYSTEM is restarted, the HARDWARE CLOCK is used to INITIALIZE the SOFTWARE CLOCK

CONFIGURE A LOOPBACK INTERFACE FOR AN NTP SERVER

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Why configure a LOOPBACK DEVICE on R1 for NTP ?

If one of R1’s ROUTER INTERFACES goes down, it will still be accessible via R3’s ROUTING path

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SET NTP SERVER for R2 using the LOOPBACK INTERFACE on R1

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SETTING R3 NTP SOURCE SERVERS using R1 and R2

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NOTE : R1 has PREFERENCE because it’s STRATUM TIER is HIGHER than R2s

CONFIGURING NTP SERVER MODE

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CONFIGURING NTP SYMMETRIC ACTIVE MODE

Command to configure NTP SYMMETRIC MODE R2(config)#ntp peer <peer ip address>

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CONFIGURE NTP AUTHENTICATION

  • NTP AUTHENTICATION can be configured, although it is OPTIONAL
  • It allows NTP CLIENTS to ensure they only sync to the intended SERVERS
  • To CONFIGURE NTP AUTHENTICATION:
    • ntp authenticate (Enables NTP AUTHENTICATION)
    • ntp authenticate-key *key-number* md5 *key* (Create the NTP AUTHENTICATION Key(s))
    • ntp trusted-key *key-number* (Specify the Trusted Key(s))
    • ntp server *ip-address* key *key-number* (Specify which key to use for the server)

EXAMPLE CONFIGURATIONS

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NTP COMMAND REVIEW

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