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Blueprint Focus: Network Time Protocol (NTP)

There are multiple reasons that NTP gets used by Cisco Unified Communications Manager.  Among the most critical is to provide time synchronization between all of the servers within the cluster - as well as with security devices,  routers, switches, gatekeepers and gateways outside of the cluster.

Without proper time synchronization, the timestamps found on all of the logs, traces and debugs available would be very difficult to align, thus making troubleshooting much more difficult.  It's also critical for the generation of proper billing records and CDR (call detail records).


Design Recommendations

By default, any CUCM subscribers that are added to the cluster will automatically synchronize their time with the publisher via NTP.  No extra configuration is necessary.

However, the publisher will use its internal clock as the basis of time for the entire cluster unless you configure an external time source.

Cisco recommends that you use an external stratum-1, stratum-2, or stratum-3 external NTP server as a time source.  This time source should use NTP version 4 AND not be running on a windows server which typically uses simple network time protocol (SNTP).  Linux-based unified call managers cannot synchronize with SNTP.

See related articles at bottom of this post for more detailed NTP information (if you are not familiar with the "stratum" concept)

Configuring CUCM (Publisher) to Use External NTP Source
  1. Choose OS Administration > Settings > NTP Server in order to navigate to the Cisco Unified Operating System Admin web interface.
  2. Configure an IP addres for a stratum-1, stratum-2, or stratum-3 NTP server.
Configuring a SIP Phone to Use an NTP Reference

Refer to this document for the details.  Note that you can only do this for SIP phones.  SCCP phones will still get their time from their call manager.

QoS Marking Recommendations for NTP

  • Layer 2 LAN CoS:  2
  • Layer 3 IP Precedence:  2
  • Layer 3 DSCP:  16
  • Layer 3 PHB:  CS2

Configuring IOS Devices to use NTP

You will want to synchronize your routing and switching devices to a valid NTP source as well for the same reasons discussed above.  This is fairly simple on IOS devices as shown below.  

ntp server

Reference Cisco IOS Fundamentals for some information on how to make your IOS device an NTP server (so you can point other devices to it for synchronization).

NTP Troubleshooting Tips

From the CUCM CLI
  1. utils ntp status
  2. utils ntp config
  3. utils network capture port 123
Issue with Time Synchronization on CUCM with Network Time Protocol Server

Key Concepts For Review
  1. Linux-based call managers cannot synchronize with SNTP (Windows Servers)
  2. Subscribers are automatically synchronized with the cluster's publisher upon installation
  3. Cisco recommends synchronizing Unified CM with a Cisco IOS or Linux-based NTP server
  4. If possible, apply the CS2 PHB (per hop behavior) to your NTP traffic within your QoS policy

Suggested Reading
  1. CUCM 7.x SRND, Network Infrastructure
  2. Cisco IP Telephony Clock Synchronization Best Practices
  3. Phone NTP Reference Configuration
  4. Wikipedia - Network Time Protocol
  5. Cisco IOS Fundamentals - Configuring NTP
  6. CUCM SRND 7.x, Full PDF

CCIE Voice Written Blueprint

The relevant section of the blueprint that includes this topic is shown below.

1.00    Infrastructure Protocols   
1.01    DNS   
1.02    TFTP   
1.03    NTP
1.04    Power over Ethernet   
1.05    Voice and Data VLAN   
1.06    Troubleshooting Infrastructure Protocols

Full Cisco CCIE Voice Written Blueprint

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