看网上介绍的比较少，拿来说一下

先介绍几条相关命令

ntp update-calendar     Periodically update calendar with NTP time

clock calendar-valid      Calendar time is authoritative

clock summer-time       Configure summer (daylight savings) time

clock timezone               Configure time zone

拓扑图如下搭建：

R1————R2

R1为server，R2为client

两台路由器开启lo 0 ，跑一个igp协议使其互通

使用lo 0做ntp

R1环回口1.1.1.1

R2环回口2.2.2.2

直连网段10.1.1.0/24

最简单的配置：

R1

ntp maste 1

ntp source loopback 0

R2

ntp server 1.1.1.1

ntp source loopback 0

做认证和ACL的配置

R1

access-list 1 permit 2.2.2.2

ntp authentication-key 1 md5 110A1016141D 7
ntp authenticate
ntp trusted-key 1
ntp source Loopback0
ntp access-group serve-only 1
ntp master 5
ntp peer 2.2.2.2 key 1

R2

access-list 1 permit 1.1.1.1

ntp authentication-key 1 md5 094F471A1A0A 7
ntp authenticate
ntp trusted-key 1
ntp clock-period 17179858
ntp source Loopback0
ntp access-group peer 1
ntp server 1.1.1.1 key 1

验证：

R2#sh ntp status
Clock is synchronized, stratum 6, reference is 10.1.1.1
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**18
reference time is C02947A9.7C5DC920 (00:19:53.485 UTC Fri Mar 1 2002)
clock offset is 4.2581 msec, root delay is 35.55 msec
root dispersion is 21.22 msec, peer dispersion is 16.94 msec

R2#sh clock detail
00:20:05.968 UTC Fri Mar 1 2002
Time source is NTP