NTP makes sure every network device has the same precise time so that logs, security and troubleshooting make sense.

Facts

Point	Detail
Purpose	Synchronize time on network devices
Transport	UDP port 123
Model	Hierarchical stratum model
Accuracy	Milliseconds (LAN), typically <100 ms over WAN/internet
Alternative	PTP (very high precision, hardware dependent)
Used for	Logs, syslog, SNMP, Kerberos, certificates, SIEM

How it works

• Stratum 0: reference clocks (GPS, atomic)
• Stratum 1: servers directly connected to stratum 0
• Stratum 2–15: clients / secondary servers
• A client sends a time request, measures delay and offset, and adjusts the clock gradually (slew) or directly (step if the difference is large). Cisco devices normally slew to avoid time jumps.

What problem does NTP solve?

• Inconsistent timestamps in logs, impossible cross-device troubleshooting, security failures (certificates, AAA, Kerberos), wrong SIEM correlation → fixed by a central time source and consistent time everywhere.

Special considerations

• UDP 123 must be allowed (ACL/firewall)
• NTP delivers UTC – local time needs clock timezone
• Use NTP authentication in enterprise, internal NTP servers instead of public ones directly
• Time may be wrong until the first NTP sync completes

Master vs server vs peer

	Master	Server	Peer
Use	No external source (lab, fallback)	Classic client → server	Active–active sync
Behaviour	Device declares itself time source	Clients receive, never reply time	Both adjust each other
Structure	—	Hierarchical	Flat

• ntp master becomes stratum 8 by default (or manual) – never use in production if real NTP exists
• ntp server is unidirectional
• ntp peer requires mutual configuration – used in core/datacenter for redundancy

Configuration

! Simple client
ntp server 192.0.2.10
! With prefer + stable source interface (best practice)
ntp server 192.0.2.10 prefer
ntp source Loopback0
! Authentication (CCNP)
ntp authentication-key 1 md5 CISCO123
ntp authenticate
ntp trusted-key 1
ntp server 192.0.2.10 key 1
! Local fallback master (stratum 5 = low priority)
ntp master 5
clock timezone CET 1
! Peer on BOTH routers
ntp peer 192.0.2.2

Verification & troubleshooting

show ntp status
show ntp associations
show clock detail

Check: Clock is synchronized, Reference is …, stratum, and the reach field. Legend: * = selected source, + = candidate, ~ = peer; stratum reveals the hierarchy.

Exam overview

Command	Function
ntp master	Local time source
ntp server x.x.x.x	Client → server
ntp peer x.x.x.x	Active–active
ntp source Lo0	Stable source IP
prefer	Primary source

CCNA: what NTP is, stratum concept, simple client config, basic verification. CCNP: authentication, peers vs servers, design (internal servers), security, troubleshooting.