Network Time Protocol

Network Time Protocol (NTP) is a protocol designed to time-synchronize a network of machines. NTP runs on User Datagram Protocol (UDP), which in turn runs on IP. NTP Version 3 is documented in RFC 1305.

This module describes how to configure Network Time Protocol on Cisco devices.

Information About Network Time Protocol

Time and Calendar Services

The primary source for time data on your system is the software clock. This clock runs from the moment the system starts up and keeps track of the current date and time. The software clock can be set from a number of sources and in turn can be used to distribute the current time through various mechanisms to other systems. When a device with a hardware clock is initialized or rebooted, the software clock is initially set based on the time in the hardware clock. The software clock can then be updated from the following sources:

  • Manual configuration (using the hardware clock)

  • Network Time Protocol (NTP)

  • Simple Network Time Protocol (SNTP)

  • Virtual Integrated Network Service (VINES) Time Service

Because the software clock can be dynamically updated, it has the potential to be more accurate than the hardware clock.

The software clock can provide time to the following services:

  • Access lists

  • Logging and debugging messages

  • NTP

  • The hardware clock

  • User show commands

  • VINES Time Service


Note

The software clock cannot provide time to the NTP or VINES Time Service if the clock was set using SNTP.

The software clock keeps track of time internally based on the Coordinated Universal Time (UTC), also known as Greenwich Mean Time (GMT). You can configure information about the local time zone and summer time (daylight saving time) so that time is displayed correctly relative to the local time zone.

The software clock keeps track of whether the time is authoritative (that is, whether it has been set by a time source considered to be authoritative). If it is not authoritative, the time will be available only for display purposes and will not be redistributed.

Network Time Protocol

Network Time Protocol (NTP) is a protocol designed to time-synchronize a network of machines. NTP runs on UDP, which in turn runs on IP. NTP Version 3 (NTPv3) is documented in RFC 1305.

An NTP network usually gets its time from an authoritative time source such as a radio clock or an atomic clock attached to a time server. NTP then distributes this time across the network. NTP is extremely efficient; no more than one packet per minute is necessary to synchronize two machines to the accuracy of within a millisecond of one another.

NTP uses the concept of a stratum to describe how many NTP hops away a machine is from an authoritative time source. A stratum 1 time server typically has an authoritative time source (such as a radio or atomic clock or a Global Positioning System [GPS] time source) directly attached, a stratum 2 time server receives its time via NTP from a stratum 1 time server, and so on.

NTP has two ways to avoid synchronizing to a machine whose time may not be accurate. NTP does not synchronize to a machine that is not in turn synchronized with the NTP. NTP compares the time reported by several machines and does not synchronize to a machine whose time is significantly different from others, even if its stratum is lower. This strategy effectively builds a self-organizing tree of NTP servers.

Our implementation of NTP does not support stratum 1 service; that is, you cannot connect to a radio or atomic clock (for some specific platforms, however, you can connect to a GPS time-source device). We recommend that the time service you derive for your network from the public NTP servers that are available in the IP Internet.

If the network is isolated from the Internet, our implementation of NTP allows a machine to be configured so that it acts as though it is synchronized via NTP, when in fact the network has determined the time by using other means. Other machines can then synchronize to that machine via NTP.

A number of manufacturers include NTP software for their host systems and a publicly available version for systems running UNIX. This software also allows UNIX-derivative servers to acquire the time directly from an atomic clock, which would subsequently propagate time information along to Cisco devices.

The communication between machines running NTP (known as associations) are usually statically configured; each machine is given the IP address of all machines with which it should form associations. Accurate timekeeping is made possible through exchange of NTP messages between each pair of machines with an association.

However, in a LAN environment, NTP can be configured to use IP broadcast messages instead. This alternative reduces configuration complexity because each machine can be configured to send or receive broadcast messages. However, the accuracy of timekeeping is marginally reduced because the information flow is only one way.

The time kept on a machine is a critical resource, so we strongly recommend that you use the security features of NTP to avoid the accidental or malicious setting of incorrect time. Two security mechanisms are available: an access-list-based restriction scheme and an encrypted authentication mechanism.

When multiple sources of time (VINES, hardware clock, manual configuration) are available, NTP is always considered to be more authoritative. NTP time overrides the time set by any other method.

NTP services are disabled on all interfaces by default.

For more information about NTP, see the following sections:

Poll-Based NTP Associations

Networking devices running NTP can be configured to operate in variety of association modes when synchronizing time with reference time sources. A networking device can obtain time information on a network in two ways—by polling host servers and by listening to NTP broadcasts. This section focuses on the poll-based association modes. Broadcast-based NTP associations are discussed in the Broadcast-Based NTP Associations section.

The following are the two most commonly used poll-based association modes:

  • Client mode

  • Symmetric active mode

The client and the symmetric active modes should be used when NTP is required to provide a high level of time accuracy and reliability.

When a networking device is operating in the client mode, it polls its assigned time-serving hosts for the current time. The networking device will then pick a host from among all the polled time servers to synchronize with. Because the relationship that is established in this case is a client-host relationship, the host will not capture or use any time information sent by the local client device. This mode is most suited for file-server and workstation clients that are not required to provide any form of time synchronization to other local clients. Use the ntp server command to individually specify the time server that you want your networking device to consider synchronizing with and to set your networking device to operate in the client mode.

When a networking device is operating in the symmetric active mode, it polls its assigned time-serving hosts for the current time and it responds to polls by its hosts. Because this is a peer-to-peer relationship, the host will also retain time-related information of the local networking device that it is communicating with. This mode should be used when a number of mutually redundant servers are interconnected via diverse network paths. Most stratum 1 and stratum 2 servers on the Internet adopt this form of network setup. Use the ntp peer command to individually specify the time serving hosts that you want your networking device to consider synchronizing with and to set your networking device to operate in the symmetric active mode.

The specific mode that you should set for each of your networking devices depends primarily on the role that you want them to assume as a timekeeping device (server or client) and the device’s proximity to a stratum 1 timekeeping server.

A networking device engages in polling when it is operating as a client or a host in the client mode or when it is acting as a peer in the symmetric active mode. Although polling does not usually place a burden on memory and CPU resources such as bandwidth, an exceedingly large number of ongoing and simultaneous polls on a system can seriously impact the performance of a system or slow the performance of a given network. To avoid having an excessive number of ongoing polls on a network, you should limit the number of direct, peer-to-peer or client-to-server associations. Instead, you should consider using NTP broadcasts to propagate time information within a localized network.

Broadcast-Based NTP Associations

Broadcast-based NTP associations should be used when time accuracy and reliability requirements are modest and if your network is localized and has more than 20 clients. Broadcast-based NTP associations are also recommended for use on networks that have limited bandwidth, system memory, or CPU resources.

A networking device operating in the broadcast client mode does not engage in any polling. Instead, it listens for NTP broadcast packets that are transmitted by broadcast time servers. Consequently, time accuracy can be marginally reduced because time information flows only one way.

Use the ntp broadcast client command to set your networking device to listen for NTP broadcast packets propagated through a network. For broadcast client mode to work, the broadcast server and its clients must be located on the same subnet. You must enable the time server that transmits NTP broadcast packets on the interface of the given device by using the ntp broadcast command.

NTP Access Group

The access list-based restriction scheme allows you to grant or deny certain access privileges to an entire network, a subnet within a network, or a host within a subnet. To define an NTP access group, use the ntp access-group command in global configuration mode.

The access group options are scanned in the following order, from least restrictive to the most restrictive:

  1. ipv4 —Configures IPv4 access lists.

  2. ipv6 —Configures IPv6 access lists.

  3. peer —Allows time requests and NTP control queries, and allows the system to synchronize itself to a system whose address passes the access list criteria.

  4. serve —Allows time requests and NTP control queries, but does not allow the system to synchronize itself to a system whose address passes the access list criteria.

  5. serve-only —Allows only time requests from a system whose address passes the access list criteria.

  6. query-only —Allows only NTP control queries from a system whose address passes the access list criteria.

If the source IP address matches the access lists for more than one access type, the first type is granted access. If no access groups are specified, all access types are granted access to all systems. If any access groups are specified, only the specified access types will be granted access.

For details on NTP control queries, see RFC 1305 (NTP Version 3).

The encrypted NTP authentication scheme should be used when a reliable form of access control is required. Unlike the access list-based restriction scheme that is based on IP addresses, the encrypted authentication scheme uses authentication keys and an authentication process to determine if NTP synchronization packets sent by designated peers or servers on a local network are deemed as trusted before the time information that they carry along with them is accepted.

The authentication process begins from the moment an NTP packet is created. Cryptographic checksum keys are generated using the message digest algorithm 5 (MD5) and are embedded into the NTP synchronization packet that is sent to a receiving client. Once a packet is received by a client, its cryptographic checksum key is decrypted and checked against a list of trusted keys. If the packet contains a matching authentication key, the time-stamp information that is contained within the packet is accepted by the receiving client. NTP synchronization packets that do not contain a matching authenticator key are ignored.


Note

In large networks, where many trusted keys must be configured, the Range of Trusted Key Configuration feature enables configuring multiple keys simultaneously.

It is important to note that the encryption and decryption processes used in NTP authentication can be very CPU-intensive and can seriously degrade the accuracy of the time that is propagated within a network. If your network setup permits a more comprehensive model of access control, you should consider the use of the access list-based form of control.

After NTP authentication is properly configured, your networking device will synchronize with and provide synchronization only to trusted time sources.

NTP Services on a Specific Interface

Network Time Protocol (NTP) services are disabled on all interfaces by default. NTP is enabled globally when any NTP commands are entered. You can selectively prevent NTP packets from being received through a specific interface by using the ntp disable command in interface configuration mode.

Source IP Address for NTP Packets

When the system sends an NTP packet, the source IP address is normally set to the address of the interface through which the NTP packet is sent. Use the ntp source interface command in global configuration mode to configure a specific interface from which the IP source address will be taken.

This interface will be used for the source address for all packets sent to all destinations. If a source address is to be used for a specific association, use the source keyword in the ntp peer or ntp server command.

System as an Authoritative NTP Server

Use the ntp command in global configuration mode if you want the system to be an authoritative NTP server, even if the system is not synchronized to an outside time source.


Note

Use the ntp primary command with caution. It is very easy to override valid time sources using this command, especially if a low stratum number is configured. Configuring multiple machines in the same network with the ntp primary command can cause instability in timekeeping if the machines do not agree on the time.

Orphan Mode

The NTP subnet is sometimes isolated from local reference clocks or Internet clock servers. During this period of isolation, the subnet servers and clients are synchronized to a common time scale. The local clock driver simulates a UTC source to provide a common time scale. A server connected to the driver directly or indirectly synchronizes the other hosts in the subnet.

Using a local clock driver may sometimes result in irrecoverable failures of the subnet, and maintaining redundancy using multiple servers is not feasible. The Orphan Mode feature, which does not have any such disadvantages, eliminates the need for a local clock driver. The Orphan Mode feature provides a single simulated UTC source with multiple servers and a seamless switching mechanism as servers recover from a failure.

In private networks, one or multiple core servers operating at the lowest stratum is normally included. You must configure each of these servers as backups for other servers using symmetric or broadcast modes. Even if one core server reaches a UTC source, the entire subnet synchronizes to the simulating server. If none of the servers reach a UTC source, one of the servers, which is known as the orphan parent, can simulate a UTC source, and serve as the simulated UTC source for all the other hosts, known as orphan children, in the subnet.

Use the ntp orphan stratum command to enable a host for orphan mode, where stratum is a stratum value less than 16 and greater than any stratum value that occurs in the configured Internet time servers. However, you must provide sufficient stratums so that every subnet host dependent on the orphan children has a stratum value less than 16. If no associations for other servers or reference clocks are configured, you must set the orphan stratum value to 1.

An orphan parent operating at stratum 1 with no sources displays the reference ID LOOP. An orphan parent not operating at stratum 1 displays the UNIX loopback address 127.0.0.1. Ordinary NTP clients use a selection metric based on delay and dispersion, whereas orphan children use a metric computed from the IP address of each core server in the subnet. Each orphan child selects the orphan parent with the smallest metric as the root server.

A server that loses all sources, continuously synchronizes the local clock driver with other servers, thus backing up the server. Enable orphan mode only in core servers and orphan children.

The following figure illustrates how orphan mode is set up, and a peer network configuration, where two primary or secondary (stratum 2) servers are configured with reference clocks or public Internet primary servers, with each using symmetric modes.

Figure 1. Orphan Mode Setup
Prerequisites for Orphan Mode

To ensure smooth function of the orphan mode, you must configure each core server with available sources to operate at the same stratum. Configure the ntp orphan command in all the core servers and the orphan children. Configure each orphan child with all root servers.

Simple Network Time Protocol

Simple Network Time Protocol (SNTP) is a simplified, client-only version of NTP. SNTP can receive only the time from NTP servers; it cannot be used to provide time services to other systems.

SNTP typically provides time within 100 milliseconds of the accurate time, but it does not provide the complex filtering and statistical mechanisms of NTP. In addition, SNTP does not authenticate traffic, although you can configure extended access lists to provide some protection. An SNTP client is more vulnerable to servers that have unexpected behavior than an NTP client, and should be used only in situations where strong authentication is not required.

You can configure SNTP to request and accept packets from configured servers or to accept NTP broadcast packets from any source. When multiple sources are sending NTP packets, the server with the best stratum is selected. (See the Network Time Protocol section on page 3 for a description of strata.) If multiple servers are at the same stratum, a configured server is preferred over a broadcast server. If multiple servers pass both tests, the first one to send a time packet is selected. SNTP will choose a new server only if it stops receiving packets from the currently selected server, or if a better server (according to the criteria described) is discovered.

VINES Time Service

Time service is available when Banyan VINES is configured. This protocol is a standard part of VINES. The Cisco implementation allows the VINES time service to be used in two ways. First, if the system has learned the time from some other source, it can act as a VINES time server and provide time to other machines running VINES. Second, it can use the VINES time service to set the software clock if no other form of time service is available.


Note

Support for Banyan VINES and Xerox Network Systems (XNS) is not available in all releases.

Hardware Clock

Some devices contain a battery-powered hardware clock that tracks the date and time across system restarts and power outages. The hardware clock is always used to initialize the software clock when the system is restarted.


Note

Within the CLI command syntax, the hardware clock is referred to as the system calendar.

If no other source is available, the hardware clock can be considered as an authoritative source of time and be redistributed via NTP. If NTP is running, the hardware clock can be updated periodically from NTP, compensating for the inherent drift, which is the consistent gain or loss of time at a certain rate if the hardware clock is left to run.

You can configure a hardware clock (system calendar) on any device to be periodically updated from the software clock. We recommend that you use this configuration for any device using NTP, because the time and date on the software clock (set using NTP) will be more accurate than the hardware clock, because the time setting on the hardware clock has the potential to drift slightly over time.

Use the ntp update-calendar command in global configuration mode if a routing device is synchronized to an outside time source via NTP and you want the hardware clock to be synchronized to NTP time.

Time Ranges

The Cisco software allows implementation of features based on the time of day. The time-range global configuration command defines specific times of the day and week, which can then be referenced by a function, so that those time restrictions are imposed on the function itself.

Depending on your release, IP and Internetwork Packet Exchange (IPX) extended access lists are the only functions that can use time ranges. The time range allows the network administrator to define when the permit or deny statements in the access list are in effect. Prior to the introduction of this feature, access list statements were always in effect once they were applied. Both named and numbered access lists can reference a time range.


Note

The time range relies on the system’s software clock. For the time range feature to work the way you intend, you need a reliable clock source. We recommend that you use NTP to synchronize the system’s software clock.

Benefits of time ranges include the following:

  • The network administrator has more control over permitting or denying a user access to resources. These resources could be an application (identified by an IP address/mask pair and a port number), policy routing, or an on-demand link (identified as interesting traffic to the dialer).

  • Network administrators can set a time-based security policy, including the following:
    • Perimeter security using the Cisco Firewall feature set or access lists.
    • Data confidentiality with Cisco Encryption Technology or IP security.

  • Policy-based routing and queueing functions are enhanced.

  • When provider access rates vary by time of day, traffic can be rerouted automatically and cost-effectively.

  • Service providers can dynamically change a committed access rate (CAR) configuration to support the quality of service (QoS) service level agreements (SLAs) that are negotiated for certain times of the day.

Network administrators can control logging messages. Access list entries can log traffic at certain times of the day, but not constantly. Therefore, administrators can deny access without the need to analyze the many logs generated during peak hours.

How to Configure Network Time Protocol

Configuring NTP

Restrictions for Network Time Protocol

The Network Time Protocol (NTP) package contains a vulnerability that could allow an unauthenticated, remote attacker to cause a denial of service (DoS) condition. NTP versions 4.2.4p7 and earlier are vulnerable.

The vulnerability is due to an error in handling of certain malformed messages. An unauthenticated, remote attacker could send a malicious NTP packet with a spoofed source IP address to a vulnerable host. The host that processes the packet sends a response packet back to the transmitter. This action could start a loop of messages between the two hosts that could cause both the hosts to consume excessive CPU resources, use up the disk space by writing messages to log files, and consume the network bandwidth. All of these could cause a DoS condition on the affected hosts.

For more information, see the Network Time Protocol Package Remote Message Loop Denial of Service Vulnerability web page.

Cisco software releases that support NTPv4 are not affected. All other versions of Cisco software are affected.

To display whether a device is configured with NTP, use the show running-config | include ntp command. If the output returns any of the following commands, then that device is vulnerable to the attack:

  • ntp broadcast client

  • ntp primary

  • ntp multicast client

  • ntp peer

  • ntp server

For more information on understanding Cisco software releases, see the White Paper: Cisco IOS and NX-OS Software Reference Guide.

There are no workarounds for this vulnerability other than disabling NTP on the device. Only packets destined for any configured IP address on the device can exploit this vulnerability. Transit traffic will not exploit this vulnerability.

Depending on your release, your feature will process NTP mode 7 packets and will display the message “NTP: Receive: dropping message: Received NTP private mode 7 packet ” if debugs for NTP are enabled. Configure the ntp allow mode private command to process NTP mode 7 packets. This command is disabled by default.


Note

NTP peer authentication is not a workaround and is a vulnerable configuration.

NTP services are disabled on all interfaces by default.

Networking devices running NTP can be configured to operate in a variety of association modes when synchronizing time with reference time sources. A networking device can obtain time information on a network in two ways: by polling host servers and by listening to NTP broadcasts.

Line Aux 0 option is disabled by default.

When you configure both IP address and FQDN of the same NTP server in Cisco IOS XE, only the FQDN configuration is displayed in the show running-config command output after FQDN resolves to the same IP address.

Configuring Poll-Based NTP Associations

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ntp peer ip-address [normal-sync ] [version number ] [key key-id ] [prefer ]
  4. ntp server ip-address [version number ] [key key-id ] [prefer ]
  5. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ntp peer ip-address [normal-sync ] [version number ] [key key-id ] [prefer ]

Example:

Device(config)# ntp peer 192.168.10.1 normal-sync version 2 prefer

Forms a peer association with another system.

Step 4

ntp server ip-address [version number ] [key key-id ] [prefer ]

Example:

Device(config)# ntp server 192.168.10.1 version 2 prefer

Forms a server association with another system.

Step 5

end

Example:

Device(config)# end

Exits global configuration mode and returns to privileged EXEC mode.

Configuring Broadcast-Based NTP Associations

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. interface type number
  4. ntp broadcast version number
  5. ntp broadcast client
  6. ntp broadcastdelay microseconds
  7. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

interface type number

Example:

Device(config)# interface GigabitEthernet 0/0

Configures an interface and enters interface configuration mode.

Step 4

ntp broadcast version number

Example:

Device(config-if)# ntp broadcast version 2

Configures the specified interface to send NTP broadcast packets.

Step 5

ntp broadcast client

Example:

Device(config-if)# ntp broadcast client

Configures the specified interface to receive NTP broadcast packets.

Step 6

ntp broadcastdelay microseconds

Example:

Device(config-if)# ntp broadcastdelay 100

Adjusts the estimated round-trip delay for NTP broadcasts.

Step 7

end

Example:

Device(config-if)# end

Exits interface configuration mode and returns to privileged EXEC mode.

Configuring NTP Authentication

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ntp authenticate
  4. ntp authentication-key number md5 key
  5. ntp authentication-key number md5 key
  6. ntp authentication-key number md5 key
  7. ntp trusted-key key-number [- end-key ]
  8. ntp server ip-address key key-id
  9. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

ntp authenticate

Example:

Device(config)# ntp authenticate

Enables the NTP Authentication feature.

Step 4

ntp authentication-key number md5 key

Example:

Defines authentication keys.

  • Each key has a key number, a type, and a value.

Step 5

ntp authentication-key number md5 key

Example:

Defines authentication keys.

  • Each key has a key number, a type, and a value.

Step 6

ntp authentication-key number md5 key

Example:

Defines authentication keys.

  • Each key has a key number, a type, and a value.

Step 7

ntp trusted-key key-number [- end-key ]

Example:

Device(config)# ntp trusted-key 1 - 3

Defines trusted authentication keys.

  • If a key is trusted, this device will be ready to synchronize to a system that uses this key in its NTP packets.

Step 8

ntp server ip-address key key-id

Example:

Device(config)# ntp server 172.16.22.44 key 2

Allows the software clock to be synchronized by an NTP time server.

Note

 

When multiple NTP servers are configured and logging is enabled, clock synchronization lost messages are randomly seen on the device. To resolve this issue, configure the NTP server using peer keyword. 

Device(config)# ntp server ip-address [version number] [key key-id] [prefer]

Step 9

end

Example:

Device(config)# end

Exits global configuration mode and returns to privileged EXEC mode.

Configuring an External Reference Clock

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. line aux line-number
  4. end
  5. show ntp associations
  6. show ntp status
  7. debug ntp refclock

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:

Device# configure terminal

Enters global configuration mode.

Step 3

line aux line-number

Example:

Device(config)# line aux 0

Enters line configuration mode for the auxiliary port 0.

Step 4

end

Example:

Device(config-line)# end

Exits line configuration mode and returns to privileged EXEC mode.

Step 5

show ntp associations

Example:

Device# show ntp associations

Displays the status of NTP associations, including the status of the GPS reference clock.

Step 6

show ntp status

Example:

Device# show ntp status

Displays the status of NTP.

Step 7

debug ntp refclock

Example:

Device# debug ntp refclock

Allows advanced monitoring of reference clock activities for the purposes of debugging.

Configuring Orphan Mode

To configure orphan mode, you would require at least two clients. The following task shows how to configure orphan mode on one client. Repeat the steps in the other client.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. ntp server ip-address
  4. ntp peer ip-address
  5. ntp orphan stratum
  6. Repeat steps 1 to 5 on the other client.

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:

Router> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:
Router# configure terminal

Enters global configuration mode.

Step 3

ntp server ip-address

Example:
Router(config)# ntp server 10.1.1.1

Forms a server association with another system.

Step 4

ntp peer ip-address

Example:
Router(config)# ntp peer 172.16.0.1

Forms a peer association with another system.

Note

 

Use an IP address that is different from the one you just configured, such as 172.16.0.2, while configuring the peer in the other client.

Step 5

ntp orphan stratum

Example:
Router(config)# ntp orphan 4

Enables orphan mode in the host.

Step 6

Repeat steps 1 to 5 on the other client.

Configuring Rate-Limiting Delay

use the following example to configure rate-limiting delay to NTP mode-6 queries: 

Router(config)#ntp allow mode control ?
  <0-15>  Rate limiting delay (s)
  <cr>    <cr>

Router(config)#ntp allow mode control 7
Router(config)#

Configuring SNTP

SNTP generally is supported on those platforms that do not provide support for NTP. SNTP is disabled by default. To configure SNTP, perform the following task.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. sntp server {address | hostname } [version number ]
  4. sntp broadcast client
  5. exit
  6. show sntp

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

sntp server {address | hostname } [version number ]

Example:


Device(config)# sntp server 192.168.2.1 version 2

Configures SNTP to request NTP packets from an NTP server.

  • Enter the sntp server command once for each NTP server. The NTP servers must be configured to respond to the SNTP messages from the device.

Step 4

sntp broadcast client

Example:


Device(config)# sntp broadcast client

Configures SNTP to accept NTP packets from any NTP broadcast server.

Note

 

If you enter both the sntp server command and the sntp broadcast client command, the device will accept time from a broadcast server but will prefer time from a configured server, assuming that the strata are equal.

Step 5

exit

Example:


Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Step 6

show sntp

Example:


Device# show sntp

Displays information about SNTP.

Configuring VINES Time Service

Time service is available when Banyan VINES is configured. This protocol is a standard part of VINES. Perform the following task to configure VINES Time Service.


Note

Depending on your release, the Banyan VINES and XNS is available in the Cisco software. The vines time set-system and vines time use-system commands are not available in some releases.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. vines time use-system
  4. vines time set-system
  5. exit

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

vines time use-system

Example:


Device(config)# vines time use-system

Distributes the system software clock time to other VINES systems.

Step 4

vines time set-system

Example:


Device(config)# vines time set-system

Sets the software clock system time and date as derived from VINES time services.

Step 5

exit

Example:


Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Configuring the Time and Date

If no other source of time is available, you can manually configure the current time and date after the system is restarted. The time will remain accurate until the next system restart. We recommend that you use manual configuration only as a last resort.

If you have an outside source to which the device can synchronize, you need not manually set the software clock. Perform the following task to configure the time and date manually.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. clock timezone zone hours-offset [minutes-offset ]
  4. clock summer-time zone recurring [week day month hh:mm week day month hh:mm [offset ]]
  5. clock summer-time zone date date month year hh:mm date month year hh:mm [offset ]
  6. exit
  7. clock set hh:mm:ss date month year

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

clock timezone zone hours-offset [minutes-offset ]

Example:


Device(config)# clock timezone PST 2 30

Configures the time zone used by the Cisco software.

Note

 

The minutes-offset argument of the clock timezone command is available for those cases where a local time zone is a percentage of an hour different from UTC/GMT. For example, the time zone for some sections of Atlantic Canada (AST) is UTC –3.5. In this case, the necessary command would be clock timezone AST –3 30 .

Step 4

clock summer-time zone recurring [week day month hh:mm week day month hh:mm [offset ]]

Example:


Device(config)# clock summer-time PST recurring 1 monday january 12:12 4 Tuesday december 12:12 120

Configures summer time (daylight saving time) in areas where it starts and ends on a particular day of the week each year.

Step 5

clock summer-time zone date date month year hh:mm date month year hh:mm [offset ]

Example:


Device(config)# clock summer-time PST date 1 january 1999 12:12 4 december 2001 12:12 120

Configures a specific summer time start and end date.

  • The offset argument is used to indicate the number of minutes to add to the clock during summer time.

Step 6

exit

Example:


Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Step 7

clock set hh:mm:ss date month year

Example:


Device# clock set 12:12:12 1 january 2011

Sets the software clock.

  • Use this command if no other time sources are available. The time specified in this command is relative to the configured time zone.

Note

 

Generally, if the system is synchronized by a valid outside timing mechanism, such as an NTP or VINES clock source, or if you have a device with a hardware clock, you need not set the software clock.

Setting the Hardware Clock

Most Cisco devices have a separate hardware-based clock in addition to the software-based clock. The hardware clock is a chip with a rechargeable backup battery that can retain the time and date information across reboots of the device.

To maintain the most accurate time update from an authoritative time source on the network, the software clock should receive time updates from an authoritative time on the network. The hardware clock should in turn be updated at regular intervals from the software clock while the system is running.

The hardware clock (system calendar) maintains time separately from the software clock. The hardware clock continues to run when the system is restarted or when the power is turned off. Typically, the hardware clock needs to be manually set only once, when the system is installed.

You should avoid setting the hardware clock if you have access to a reliable external time source. Time synchronization should instead be established using NTP.

Perform the following task to set the hardware clock.

Before you begin


Note

Depending on your release, NTP runs within IOS daemon (IOSd), which updates the time on the Linux kernel. As the Linux kernel updates the hardware clock every 11 minutes, NTP does not interact with the hardware clock directly. So, the calendar-related commands are not required.

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. clock calendar-valid
  4. exit
  5. clock read-calendar
  6. clock update-calendar
  7. show calendar
  8. show clock [detail ]
  9. show ntp associations [detail ]
  10. show ntp status
  11. show sntp

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

clock calendar-valid

Example:


Device(config)# clock calendar-valid

Enables the device to act as a valid time source to which network peers can synchronize.

  • By default, the time maintained on the software clock is not considered to be reliable and will not be synchronized with NTP or VINES time service. To set the hardware clock as a valid time source, use this command.

Step 4

exit

Example:


Device(config)# exit

Exits global configuration mode and returns to privileged EXEC mode.

Step 5

clock read-calendar

Example:


Device# clock read-calendar

Sets the software clock to the new hardware clock setting.

Step 6

clock update-calendar

Example:


Device# clock update-calendar

Updates the hardware clock with a new software clock setting.

Step 7

show calendar

Example:


Device# show calendar

Displays the current hardware clock time.

Step 8

show clock [detail ]

Example:


Device# show clock detail

Displays the current software clock time.

Step 9

show ntp associations [detail ]

Example:


Device# show ntp associations detail

Displays the status of NTP associations.

Step 10

show ntp status

Example:


Device# show ntp status

Displays the status of NTP.

Step 11

show sntp

Example:


Device# show sntp

Displays information about SNTP.

Configuring Time Ranges

SUMMARY STEPS

  1. enable
  2. configure terminal
  3. time-range time-range-name
  4. Enter one of the following:
    • absolute [start hh:mm date month year ] [end hh:mm date month year ]
    • periodic day-of-the-week hh:mm to [day-of-the-week ] hh:mm
  5. end

DETAILED STEPS

  Command or Action Purpose

Step 1

enable

Example:


Device> enable

Enables privileged EXEC mode.

  • Enter your password if prompted.

Step 2

configure terminal

Example:


Device# configure terminal

Enters global configuration mode.

Step 3

time-range time-range-name

Example:


Device(config)# time-range range1

Assigns a name to the time range to be configured and enters time range configuration mode.

Step 4

Enter one of the following:

  • absolute [start hh:mm date month year ] [end hh:mm date month year ]
  • periodic day-of-the-week hh:mm to [day-of-the-week ] hh:mm

Example:


Device(config-time-range)# absolute start 12:12 30 January 1999 end 12:12 30 December 2000

Device(config-time-range)# periodic monday 12:12 to friday 12:12

Specifies when the time range will be in effect.

  • Use a combination of these commands; multiple periodic commands are allowed; only one absolute command is allowed.

Step 5

end

Example:


Device(config-time-range)# end

Exits time range configuration mode and returns to privileged EXEC mode.

Verifying Network Time Protocol

SUMMARY STEPS

  1. show calendar
  2. show clock [detail ]
  3. show ntp associations detail
  4. show ntp status
  5. show running-config | i-ntp

DETAILED STEPS

Step 1

show calendar

This command displays the current hardware clock time. The following is sample output from this command.

Example:


Device# show calendar

18:34:29 UTC Tue Jan 4 2011

Step 2

show clock [detail ]

This command displays the current software clock time. The following is sample output from this command.

Example:


Device# show clock detail

*18:38:21.655 UTC Tue Jan 4 2011
Time source is hardware calendar

Step 3

show ntp associations detail

This command displays the status of NTP associations. The following is sample output from this command.

Example:


Device# show ntp associations detail

192.168.10.1 configured, insane, invalid, unsynced, stratum 16
ref ID .INIT., time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
our mode active, peer mode unspec, our poll intvl 64, peer poll intvl 1024
root delay 0.00 msec, root disp 0.00, reach 0, sync dist 15940.56
delay 0.00 msec, offset 0.0000 msec, dispersion 15937.50
precision 2**24, version 4
org time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
rec time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
xmt time D0CDE881.9A6A9005 (18:42:09.603 UTC Tue Jan 4 2011)
filtdelay =     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
filtoffset =    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
filterror =  16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0
minpoll = 6, maxpoll = 10
192.168.45.1 configured, insane, invalid, unsynced, stratum 16
ref ID .INIT., time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
our mode client, peer mode unspec, our poll intvl 64, peer poll intvl 1024
root delay 0.00 msec, root disp 0.00, reach 0, sync dist 16003.08
delay 0.00 msec, offset 0.0000 msec, dispersion 16000.00
precision 2**24, version 4
org time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
rec time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
xmt time 00000000.00000000 (00:00:00.000 UTC Mon Jan 1 1900)
filtdelay =     0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
filtoffset =    0.00    0.00    0.00    0.00    0.00    0.00    0.00    0.00
filterror =  16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0 16000.0
minpoll = 6, maxpoll = 10

Step 4

show ntp status

This command displays the status of NTP. The following is sample output from this command.

Example:


Device# show ntp status

Clock is synchronized, stratum 8, reference is 127.127.1.1
nominal freq is 250.0000 Hz, actual freq is 250.0000 Hz, precision is 2**10
reference time is D25AF07C.4B439650 (15:26:04.294 PDT Tue Oct 21 2011)
clock offset is 0.0000 msec, root delay is 0.00 msec
root dispersion is 2.31 msec, peer dispersion is 1.20 msec
loopfilter state is 'CTRL' (Normal Controlled Loop), drift is 0.000000000 s/s
system poll interval is 16, last update was 10 sec ago.

Step 5

show running-config | i-ntp

This command displays the configured rate-limiting delay. The following is sample output from this command, with rate-limiting delay set to 7 seconds:

Example:

device# show running-config | i ntp
ntp allow mode control 7

device# show running-config all | i ntp mode
ntp allow mode control 7

Configuration Examples for Network Time Protocol

Example: Configuring Network Time Protocol

In the following example, a device with a hardware clock that has server associations with two other systems sends broadcast NTP packets, periodically updates the hardware clock, and redistributes time into VINES: 


clock timezone PST -8
clock summer-time PDT recurring

ntp server 192.168.13.57
ntp server 192.168.11.58
interface GigabitEthernet 0/0
 ntp broadcast
vines time use-system

In the following example, a device with a hardware clock has no outside time source, so it uses the hardware clock as an authoritative time source and distributes the time via NTP broadcast packets: 


clock timezone MET 2
clock calendar-valid
ntp master
interface vlan 3
 ntp broadcast

The following example shows Line Aux 0 option is disabled by default.

config-register 0x0
reload
rommon 1 > set
rommon 2 >  AUX_PORT=1
rommon 3 > SYNC
rommon 4 > reset
rommon 1 > set
rommon 2 > confreg 0x2102
rommon 3 > reset

Additional References for Network Time Protocol

Related Documents

Related Topic

Document Title

Basic System Management commands

Basic System Management Command Reference

NTP4 in IPv6

Cisco IOS Basic System Management Guide

IP extended access lists

Cisco IOS IP Addressing Configuration Guide

IPX extended access lists

Novell IPX Configuration Guide

NTP package vulnerability

Network Time Protocol Package Remote Message Loop Denial of Service Vulnerability

Cisco IOS and NX-OS software releases

'White Paper: Cisco IOS and NX-OS Software Reference Guide

Standards and RFCs

Standard/RFCs

Title

RFC 1305

Network Time Protocol (Version 3) Specification, Implementation and Analysis

Technical Assistance

Description

Link

The Cisco Support and Documentation website provides online resources to download documentation, software, and tools. Use these resources to install and configure the software and to troubleshoot and resolve technical issues with Cisco products and technologies. Access to most tools on the Cisco Support and Documentation website requires a Cisco.com user ID and password.

http://www.cisco.com/cisco/web/support/index.html

Feature Information for Network Time Protocol

The following table provides release information about the feature or features described in this module. This table lists only the software release that introduced support for a given feature in a given software release train. Unless noted otherwise, subsequent releases of that software release train also support that feature.

Use Cisco Feature Navigator to find information about platform support and Cisco software image support. To access Cisco Feature Navigator, go to www.cisco.com/go/cfn. An account on Cisco.com is not required.
Table 1. Feature Information for Network Time Protocol

Feature Name

Releases

Feature Information

Network Time Protocol

11.2(1)

12.2(28)SB

12.2(33)SRA

12.2(33)SXI

12.2(33)SXJ

12.2(50)SY

12.2(58)SE

15.0(1)M

15.1(2)S

15.1(2)SG

Cisco IOS XE Release 3E

NTP is a protocol designed to time-synchronize a network of machines. NTP runs on UDP, which in turn runs on IP. NTP is documented in RFC 1305.

The following commands were introduced or modified: ntp access-group , ntp allow mode passive , ntp authenticate , ntp authentication-key , ntp broadcast, ntp broadcast client , ntp broadcastdelay , ntp clear drift , ntp clock-period , ntp disable , ntp logging , ntp primary , ntp max-associations , ntp multicast , ntp multicast client , ntp server , ntp source , ntp trusted-key and ntp update-calendar .