Network Time Protocol
NTP Version 3 (RFC 1305) allows IP hosts to accord their time-of-day clocks with a
common antecedent clock. For instance, routers and switches can accord their clocks to make
event alternation from an SNMP administration base added meaningful, by ensuring that any
events and accessories accept authentic time stamps.
By design, best routers and switches use NTP applicant mode, adjusting their clocks based on the
time as accepted by an NTP server. NTP defines the letters that breeze amid applicant and server,
and the algorithms a applicant uses to acclimatize its clock. Routers and switches can additionally be configured as
NTP servers, as able-bodied as application NTP symmetric alive mode—a approach in which the router or switch
mutually synchronizes with addition NTP host.
NTP servers may advertence added NTP servers to access a added authentic alarm antecedent as defined
by the degree akin of the ultimate antecedent clock. For instance, diminutive clocks and Global
Positioning System (GPS) accessory transmissions accommodate a antecedent of degree 1 (lowest/best
possible degree level). For an action network, the routers and switches can accredit to a lowstratum
NTP antecedent on the Internet, or purpose-built rack-mounted NTP servers, with built-in
GPS capabilities, can be deployed.
Example 5-3 shows a sample NTP agreement on four routers, all administration the aforementioned 10.1.1.0/24
Ethernet subnet. Router R1 will be configured as an NTP server. R2 acts as an NTP changeless client
by advantage of the changeless agreement referencing R1’s IP address. R3 acts as an NTP broadcast
client by alert for R1’s NTP broadcasts on the Ethernet. Finally, R4 acts in NTP symmetric
active mode, configured with the ntp associate command.
Example 5-3 NTP Configuration
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
! First, R1’s configuration, the ntp advertisement command beneath interface fa0/0
! causes NTP to advertisement NTP updates on that interface. The aboriginal three of the
! four all-around NTP commands configure authentication; these commands are identical
! on all the routers.
R1# appearance running-config
interface FastEthernet0/0
ntp broadcast
!
ntp authentication-key 1 md5 
1514190900 7
ntp authenticate
ntp trusted-key 1
ntp adept 7
! Below, the “127.127.7.1” characters implies that this router is the NTP clock
! source. The alarm is synchronized, with degree akin 7, as configured on the
! ntp adept 7 command above.
R1# appearance ntp associations
address ref alarm st back poll ability adjournment account disp
*~127.127.7.1 127.127.7.1 6 22 64 377 0.0 0.00 0.0
* adept (synced), # adept (unsynced), + selected, - candidate, ~ configured
R1# appearance ntp status
Clock is synchronized, degree 7, advertence is 127.127.7.1
nominal freq is 249.5901 Hz, absolute freq is 249.5901 Hz, attention is 2**16
reference time is C54483CC.E26EE853 (13:49:00.884 UTC Tue Nov 16 2004)
clock account is 0.0000 msec, basis adjournment is 0.00 msec
root burning is 0.02 msec, associate burning is 0.02 msec
! R2 is configured beneath as an NTP changeless client. Note that the ntp clock-period
! command is automatically generated as allotment of the synchronization process, and
! should not be added to the agreement manually.
R2# appearance run | activate ntp
ntp authentication-key 1 md5 1514190900 7
ntp authenticate
ntp trusted-key 1
ntp clock-period 17208144
ntp server 10.1.1.1
end
! Next, R3 has been configured as an NTP advertisement client. The ntp advertisement client
! command on R3 tells it to accept for the broadcasts from R1. This configuration
! relies on the ntp advertisement subcommand on R1’s Fa0/0 interface, as apparent at the
! alpha of this example.
R3# appearance run
interface Ethernet0/0
ntp advertisement client
! R4’s agreement is listed, with the ntp peer
! command implying the use of symmetric alive mode.
R4# appearance run | beg ntp
ntp authentication-key 1 md5 0002010300 7
ntp authenticate
ntp trusted-key 1
ntp clock-period 17208233
ntp associate 10.1.1.1