How the Network Time Protocol (NTP) architecture works is that the NTP clients change their time in sync to the NTP server, but the NTP server does not change its time to reflect the NTP clients. External NTP servers are commonly used, and usually, two devices query different external NTP sources, and then the two devices peer their local stratum.
Each of the NTP peer act as a NTP client and NTP server of each other. The NTP peers try to merge their time. The NTP peers also act as backup devices in case one of them fails. They are configured to have one or more time sources, reference clocks, and higher stratum servers. So if an NTP peer loses all of its reference clocks or it fails, then the other peer will still provide time to the clients on the network.
NTP Peer Configuration
For our example, we have two servers at Stratum 1. NTP Server1 is at 10.10.10.1, and NTP Server2 is at 10.10.20.1. Our two routers, R1 and R2, are Stratum 2. R1 is an NTP client to NTP server 10.10.10.1 and R2 is an NTP client to NTP server 10.10.20.1. We will configure the two routers so that they will become NTP peers with one another.
First, we will set the NTP server for the routers using the ‘ntp server’ command. Then, the NTP peers are configured with the following command:
ntp peer ip-address
We will peer using the routers’ loopback interface addresses for our example.
R1>enable R1#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R1(config)#ntp sever 10.10.10.1 R1(config)#ntp peer 192.168.20.1
R2>enable R2#configure terminal Enter configuration commands, one per line. End with CNTL/Z. R2(config)#ntp server 10.10.20.1 R2(config)#ntp peer 192.168.10.1
To verify the NTP peer configuration, we can use the ‘show ntp associations’ and ‘show ntp status’ commands. The ‘show ntp associations’ command will show us if the clock is synchronized and the connected peers. The ‘show ntp status’ command also will tell us if the clock is synchronized or not, the stratum, and the reference clock.
NTP peering can take a lot of time to synchronize since both NTP peers treat each other as equal, as opposed to a higher stratum. Each peer adjusts at two minutes per query at maximum, so large discrepancies take time to be corrected.
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