一、安裝環境：
CentOS 7 Linux version 3.10.0-229.el7.x86_64
ntp-4.2.6p5-19.el7.centos.x86_64
二、安裝及配置
2.1 安裝
NTP可以直接通過yum來安裝，使用 rpm -qa ntp可以查看是否已安裝ntp服務，如有安裝可使用yum install ntp 來升級，當然也可以不用升級。
命令：
yum install ntp #安裝NTP服務器
2.2 配置
cat /etc/ntp.conf
# For more information about this file, see the man pages
# ntp.conf(5), ntp_acc(5), ntp_auth(5), ntp_clock(5), ntp_misc(5), ntp_mon(5).
driftfile /var/lib/ntp/drift #默認配置，無需改動
# Permit time synchronization with our time source, but do not
# permit the source to query or modify the service on this system.
restrict default nomodify notrap nopeer noquery #默認配置，無需改動
# Permit all access over the loopback interface. This could
# be tightened as well, but to do so would effect some of
# the administrative functions.
restrict 127.0.0.1 #默認配置，無需改動
restrict ::1 #默認配置，無需改動
# Hosts on local network are less restricted.
#restrict 192.168.1.0 mask 255.255.255.0 nomodify notrap
restrict 192.168.30.0 mask 255.255.255.0 nomodify notrap #增加這一行，允許哪些IP 地址可以同步NTP服務器，也可以不用配置，默認所有網段
# Use public servers from the pool.ntp.org project.
# Please consider joining the pool (http://www.pool.ntp.org/join.html).
server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst
#默認配置，可以不用改動，在中國大陸會默認使用大陸的主NTP服務器，在香港就會使用香港的NTP服務器。當然也可以註釋上面四行，自己添加，如 server 0.210.72.145.44
#broadcast 192.168.1.255 autokey# broadcast server
#broadcastclient# broadcast client
#broadcast 224.0.1.1 autokey# multicast server
#multicastclient 224.0.1.1# multicast client
#manycastserver 239.255.254.254# manycast server
#manycastclient 239.255.254.254 autokey # manycast client
# Enable public key cryptography.
#crypto
includefile /etc/ntp/crypto/pw #默認配置，無需改動
# Key file containing the keys and key identifiers used when operating
# with symmetric key cryptography.
keys /etc/ntp/keys #默認配置，無需改動
# Specify the key identifiers which are trusted.
#trustedkey 4 8 42
# Specify the key identifier to use with the ntpdc utility.
#requestkey 8
# Specify the key identifier to use with the ntpq utility.
#controlkey 8
# Enable writing of statistics records.
#statistics clockstats cryptostats loopstats peerstats
# Disable the monitoring facility to prevent amplification attacks using ntpdc
# monlist command when default restrict does not include the noquery flag. See
# CVE-2013-5211 for more details.
# Note: Monitoring will not be disabled with the limited restriction flag.
disable monitor #默認配置，無需改動
2.3 啓動NTP服務
命令：
systemctl start ntpd.service 開啓服務
systemctl stop ntpd.service 停止服務
三、測試
命令：
ntpstat #查看ntp服務器是否連通，啓動服務後需要三五分鐘後纔會連通
ntpq -p #查看ntp服務器連接狀態
netstat -tunlp | grep ntpd #查看服務端口
在另一臺linux上測試ntp同步是否可用
命令：
yum install ntpdate
ntpdate 172.31.251.196
四、交換機同步
配置命令：
conf t
ntp server 172.31.251.196
謝謝你打開這篇博文，並一直堅持看到了這裏，如果覺得對你有幫助，請不要吝嗇點一下右下角的贊。