Interpreting NTP and PTP Configurations

What it is

Network devices need the same time so that logs, certificates and troubleshooting make sense.

The Network Time Protocol (NTP) keeps clocks in step across an IP network, usually to within a few milliseconds. The Precision Time Protocol (PTP) is for cases that need much finer accuracy, such as financial trading, broadcast video and industrial control. Both work by comparing a device clock with a better one.

How it works

NTP builds a tree of servers, and PTP builds a master and follower hierarchy.

NTP stratum

The stratum says how far a clock is from a reference clock. A reference clock is stratum 0. A server directly attached to it is stratum 1.

Each hop away adds one, and a lower stratum is closer to the source and is preferred. Stratum 16 means that the clock is not synchronised. NTP uses UDP port 123.

NTP modes

Configuring an NTP client

R1(config)# ntp server 192.0.2.10
R1(config)# ntp server 192.0.2.11 prefer
R1(config)# clock timezone GMT 0
R1(config)# ntp source Loopback0

The ntp server command makes the router a client of that server. The keyword prefer marks the favoured server when two are of similar quality.

Using more than one server gives you a backup. The ntp source command sets the source interface for NTP packets, so the server sees a predictable address.

Making a router a time source

A router with no outside source can act as its own authority. This is useful in a lab, but should not be used in production without care.

R2(config)# ntp master 3

The number is the stratum that the router claims. Use a higher number than any real source, so that a real server wins if one is available.

NTP authentication

Authentication stops a device from accepting time from an unknown source.

R1(config)# ntp authenticate
R1(config)# ntp authentication-key 1 md5 MYKEY
R1(config)# ntp trusted-key 1
R1(config)# ntp server 192.0.2.10 key 1

The key number and the key text must match on the server. The trusted-key command tells the client which keys it may accept.

Verifying NTP

R1# show ntp status
R1# show ntp associations
R1# show clock detail

show ntp status should say that the clock is synchronised, and it shows the stratum and the reference server. show ntp associations lists the servers, with a mark in the first column.

If the status says unsynchronised, check IP reachability to the server, the key settings, and any access list that blocks UDP 123. Synchronisation can take a few minutes after the configuration is applied.

PTP basics

PTP follows IEEE 1588. It can reach accuracy in the sub microsecond range, because the hardware adds timestamps as messages leave and arrive. NTP does this in software, which limits its precision.

PTP devices have clear roles.

The devices choose the grandmaster with the best master clock algorithm, based on clock quality and priority values. PTP uses message exchanges with names such as Sync, Follow Up, Delay Request and Delay Response. It uses UDP ports 319 and 320 over IP.

Boundary and transparent clocks matter because switches add delay that varies with load. A boundary clock removes that error by acting as its own source. A transparent clock measures the delay and corrects for it.