Restricting NTP with an access-group

Bundle lab · $29.99

Configure a Cisco IOS router as an authoritative NTP master and restrict which clients it will serve using an NTP access-group with a standard ACL. One shared LAN (no routing) connects three routers through a Layer-2 switch. Only R2 is authorized to receive time from R1; R3 is denied. Learners deploy, verify, and troubleshoot the access-group behavior.

Track

CCNA

Est. time

40 minutes

Difficulty

Intermediate

Topics

NTP, Cisco IOS, CCNP, Security, Access Control

Learning objectives

Configure an IOS router as an authoritative NTP master with a chosen stratum

Build a standard ACL to authorize specific NTP clients

Apply ntp access-group serve-only <acl> to restrict server responses

Point NTP clients at the master and interpret show ntp associations

Explain NTP access-group modes: serve, serve-only, peer, query-only

Troubleshooting focus

NTP client points to wrong server IP or is on a different subnet

ACL number, type, or ACE is incorrect so all clients are denied

Using serve instead of serve-only changes behavior and allows peering

Expecting immediate synchronization; NTP convergence takes time

Misreading show ntp associations flags or relying on reachability alone

Topology

Own this lab's topology — and every lab — in NTP & Network Time.

Unlock with NTP & Network Time

Restricting NTP with an access-group

Learning objectives

Troubleshooting focus

Topology

Related labs

Study this topic

Grade your work