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

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

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

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

MANUAL TIME CONFIGURATION

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

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

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

CONFIGURING THE TIME ZONE

You can configure the time zone with the clock timezone command

DAYLIGHT SAVING TIME (SUMMER TIME)

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

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

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

(Peering with Devices is called …)

An NTP CLIENT can SYNC to MULTIPLE NTP SERVERS

NTP CONFIGURATION

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

(To show configuration servers)

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

st = Stratum Tier

(To show NTP Status)

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

(To show NTP clock details)

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

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

SET NTP SERVER for R2 using the LOOPBACK INTERFACE on R1

SETTING R3 NTP SOURCE SERVERS using R1 and R2

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

CONFIGURING NTP SERVER MODE

CONFIGURING NTP SYMMETRIC ACTIVE MODE

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

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

NTP COMMAND REVIEW