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Network Time Protocol NTP is a protocol to synchronize time on network  of machines.This is done getting time from an authoritative time source  such as Atomic Clock or Radio clock attached to the time server.This  time is then distributed across the network and this is being done so  efficiently where in a network having 2 devices will have the time  updated to the accuracy of within a millisecond of one another.NTP runs  on UPD which in turn run on IPv4.


NTPv4 is an extension to the current NTPv3 where it  supports both IPv4 and IPv6 and is backward compatible with NTPv3. Some  of main features of NTPv4 are discussed here:


IPv6 Support


NTPv4  extends its support for IPv6 there by enabling NTP time synchronization  possible over IPv6.Unlike NTPv3 which uses IPv4 broadcast message to  send and receive clock updates, the NTPv4 for IPv6 uses IPv6 Multicast  multicast messages for synchronization of clocks.


Enhanced Security


NTPv4  uses Symmetric Key Authentication which provides a strong cryptographic  authentication mechanism which efficiently prevents the local router  from being wrongly updated with incorrect time by any intruder. In this  NTP symmetric Key Authentication, the keys are distributed manually on  the NTP devices across the network.Note that NTPv4 group functionality  supports both IPv6 named access list and IPv4 numbered access list.


DNS Support For IPv6


In  NTPv3, the host names are resolved in to IPv4 addresses and the  resolved IPv4 addresses are  kept in memory stored in NVRAM but the host  names are lost.In case of NTPv4, the host names are also stored in  memory so that it can be readable and the configurations saved with host  names are still readable by NTPv3.

How to Configure NTPv4 in IPv6

Summary Steps:

In Configuration mode:


To enable the client software clock to be synchronized by the NTP time server, use the command ntp server <IPv6 address> version 4


To configure the symmetric active mode where the clients will be able to synchronize the time from its peer use the command

ntp peer <IPv6 address of the peer> version 4


In order to enable NTPv4 authentication, configure the below commands


ntp authenticate enables NTPv4 authentication


ntp authentication-key number md5 value  defines  authentication key for NTPv4.


ntp trusted-key key-number authenticates the identity of a system to which NTPv4 will synchronize


Useful Show Commands


The following show commands can be used to display the cache content and cache statistics


  • show clock <detail>

This command displays the date and time from the system software clock and the keyword


is used to check the clock source i.e. through NTP or hardware clock etc..Example:

Router> show clock detail
*12:40:55.911 PST Thu Nov 29 2012
Time source is NTP

  • show ntp associations <detail>


R1#show ntp association detail

2001:DB8:AA10::2 configured, insane, invalid, unsynced, stratum 16

ref ID .INIT., time D461D902.5B2530A4 (12:52:18.356 PST Thu Nov 29 2012)

our mode client, peer mode unspec, our poll intvl 64, peer poll intvl 1024

root delay 0.00 msec, root disp 0.00, reach 0, sync dist 15940.75

delay 0.00 msec, offset 0.0000 msec, dispersion 15937.50

precision 2**24, version 4

org time AFE252E2.3AC0E887 (00:12:34.229 PDT Tue Oct 4 2011)

rec time D461D902.5B2530A4 (12:52:18.356 PST Thu Nov 29 2012)

xmt time D461D4E5.852FF670 (12:34:45.520 UTC Thu Nov 29 2012)

filtdelay =     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00

filtoffset =    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00

filterror =  16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0

minpoll = 6, maxpoll = 10


  • show ntp status

Use this command to display the status of NTP

Router#show ntp status
Clock is synchronized, stratum 8, reference is
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**24
reference time is D461D902.5B2530A4 (12:52:18.356 PST Thu Nov 29 2012)
clock offset is 0.0000 msec, root delay is 0.00 msec
root dispersion is 937.73 msec, peer dispersion is 937.67 msec
loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000000000 s/s
system poll interval is 16, last update was 5 sec ago.


Note: To debug various NTP functions such as authentication validity and ntp events use the command debug ntp.



Related Information

Implementing NTPv4 in IPv6

NTPv4 : Common Issues and Troubleshooting

Network Time Protocol (NTP) - Home Page

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