Configuring NTP

This chapter describes how to configure the Network Time Protocol (NTP) on Cisco NX-OS devices.

This chapter includes the following sections:

About NTP

The Network Time Protocol (NTP) synchronizes the time of day among a set of distributed time servers and clients so that you can correlate events when you receive system logs and other time-specific events from multiple network devices. NTP uses the User Datagram Protocol (UDP) as its transport protocol. All NTP communications use Coordinated Universal Time (UTC).

An NTP server usually receives its time from an authoritative time source, such as a radio clock or an atomic clock attached to a time server, and 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 within a millisecond of each other.

NTP uses a stratum to describe the distance between a network device and an authoritative time source:

  • A stratum 1 time server is directly attached to an authoritative time source (such as a radio or atomic clock or a GPS time source).
  • A stratum 2 NTP server receives its time through NTP from a stratum 1 time server.

Before synchronizing, NTP compares the time reported by several network devices and does not synchronize with one that is significantly different, even if it is a stratum 1. Because Cisco NX-OS cannot connect to a radio or atomic clock and act as a stratum 1 server, we recommend that you use the public NTP servers available on the Internet. If the network is isolated from the Internet, Cisco NX-OS allows you to configure the time as though it were synchronized through NTP, even though it was not.


Note

You can create NTP peer relationships to designate the time-serving hosts that you want your network device to consider synchronizing with and to keep accurate time if a server failure occurs.

The time kept on a device 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 mechanisms are available: an access list-based restriction scheme and an encrypted authentication mechanism.

NTP Associations

An NTP association can be one of the following:

  • A peer association—The device can either synchronize to another device or allow another device to synchronize to it.

  • A server association—The device synchronizes to a server.

You need to configure only one end of an association. The other device can automatically establish the association.

NTP as a Time Server

The Cisco NX-OS device can use NTP to distribute time. Other devices can configure it as a time server. You can also configure the device to act as an authoritative NTP server, enabling it to distribute time even when it is not synchronized to an outside time source.

Clock Manager

Clocks are resources that need to be shared across different processes. Multiple time synchronization protocols, such as NTP, might be running in the system.

The clock manager allows you to specify the protocol to control the various clocks in the system. Once you specify the protocol, the system clock starts updating. For information on configuring the clock manager, see the Cisco Nexus 9000 Series NX-OS Fundamentals Configuration Guide.

High Availability

Stateless restarts are supported for NTP. After a reboot or a supervisor switchover, the running configuration is applied. For more information on high availability, see the Cisco Nexus 9000 Series NX-OS High Availability and Redundancy Guide.

You can configure NTP peers to provide redundancy in case an NTP server fails.

Prerequisites for NTP

NTP has the following prerequisites:

  • To configure NTP, you must have connectivity to at least one server that is running NTP.

Guidelines and Limitations for NTP

NTP has the following configuration guidelines and limitations:

  • NTP server functionality is supported.

  • Before configuring a name based NTP server (FQDN) in a non-default VRF, you must configure a DNS server under that specific VRF. If you configure the DNS server from the global configuration mode using use-vrf option, then that name based NTP server configuration will not be added to the running configuration. If you attempted to configure NTP server using this method, you must remove the NTP configuration using the no version of the command, add the DNS server under that VRF, and then add name based NTP server to the VRF. The configured DNS server must be reachable and must return the correct IP for the FQDN of the NTP server when queried.

  • We recommend that you configure a peer association with another device only when you are sure that your clock is reliable (which means that you are a client of a reliable NTP server).

  • A peer that is configured alone takes on the role of a server and should be used as a backup. If you have two servers, you can configure several devices to point to one server and the remaining devices to point to the other server. You can then configure a peer association between these two servers to create a more reliable NTP configuration.

  • If you have only one server, we recommend that you configure all the devices as clients to that server.

  • You can configure up to 64 NTP entities (servers and peers).

  • If you configure NTP in a VRF, ensure that the NTP server and peers can reach each other through the configured VRFs.

  • Manually distribute NTP authentication keys on the NTP server and Cisco NX-OS devices across the network.

  • If you are using the switch as an edge device and want to use NTP, we recommend using the ntp access-group command and filtering NTP only to the required edge devices.

  • If the system has been configured with the ntp passive , ntp broadcast client , or ntp multicast client commands, when NTP receives an incoming symmetric active, broadcast, or multicast packet, it can set up an ephemeral peer association in order to synchronize with the sender.


    Note
    Make sure that you specify ntp authenticate before enabling any of the preceding commands. Failure to do so will allow your device to synchronize with any device that sends one of the preceding packet types, including malicious attacker-controlled devices.

  • If you specify the ntp authenticate command, when a symmetric active, broadcast, or multicast packet is received, the system does not synchronize to the peer unless the packet carries one of the authentication keys that are specified in the ntp trusted-key global configuration command.

  • To prevent synchronization with unauthorized network hosts, the ntp authenticate command should be specified any time the ntp passive , ntp broadcast client , or ntp multicast client command has been specified unless other measures, such as the ntp access-group command, have been taken to prevent unauthorized hosts from communicating with the NTP service on the device.

  • The ntp authenticate command does not authenticate peer associations that are configured via the ntp server and ntp peer configuration commands. To authenticate the ntp server and ntp peer associations, specify the key keyword.

  • A maximum of four IP ACLs can be configured for a single NTP access group. IPv4 and IPv6 ACLs are supported.

  • If packet flooding occurs on the inband ports, it can increase the CPU usage by NTPD to more than 90%. To overcome this high CPU usage by NTPD, use the custom CoPP policy to rate limit the incoming traffic to NTP. For more information about creating a custom CoPP policy, refer to the Configuring Control Plane Policing chapter in the relevant version of the Cisco Nexus 9000 Series NX-OS Security Configuration Guide on cisco.com.


    Note

    The recommended rate limit is 1000 kbps for the policy CIR field and 64,000 bytes for the BC field.

  • Beginning with Cisco NX-OS Release 10.1(1), Cisco Nexus 9000 switches do not sync with stratum 14 and 15.

  • Beginning with Cisco NX-OS Release 10.3(3)F, in line with the RFC 8573 standards, NTP security is enhanced with the AES128CMAC authentication mechanism along with Type-6 encryption support for authentication keys. The following guidelines and limitations are applicable:

    • This feature provides an option to set the password as Type-0, Type-7, or Type-6.

    • The maximum number of unique keys which can be configured is 1024, and the range is 1 to 65535.

    • For Type-6 authentication to work, configure the same primary (master) key on the device, which was used to generate the new Type-6 key being configured, along with feature password encryption aes .

    • If you enforce re-encryption using the encryption re-encrypt obfuscated command, all the NTP non-Type-6 passwords will be re-encrypted to Type-6.

    • The encryption delete type6 command deletes all the Type-6 passwords configured on NTP.

    • The encryption decrypt type6 command decrypts the existing configured Type-6 passwords.

    • To perform ISSD from AES128CMAC/Type-6 supported version to a non-AES128CMAC/Type-6 supported version, unconfigure the Type-6 keys and then perform the ISSD.

    • It is recommended to specify the encryptType and keyString when programmatically (restconf/Netconf and so on) configuring a key chain. If not specified, then the key chain infra uses the already available (or default) value of the missing property to configure the keyString.

    • If you need to configure with a property missing, then you need to follow the same sequence of steps in both the peer routers.

Default Settings for NTP

The following table lists the default settings for NTP parameters.

Parameters Default
NTP Enabled
NTP authentication Disabled
NTP access Enabled
NTP logging Disabled

Configuring NTP


Note

Be aware that the Cisco NX-OS commands for this feature may differ from those commands used in Cisco IOS.

Enabling or Disabling NTP

You can enable or disable NTP. NTP is enabled by default.

SUMMARY STEPS

  1. configure terminal
  2. [no] feature ntp
  3. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] feature ntp

Example:

switch(config)# feature ntp

Enables or disables NTP.

Step 3

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
(Optional)

Copies the running configuration to the startup configuration.

Configuring the Device as an Authoritative NTP Server

You can configure the device to act as an authoritative NTP server, enabling it to distribute time even when it is not synchronized to an existing time server.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp master [stratum]
  3. (Optional) show running-config ntp
  4. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp master [stratum]

Example:

switch(config)# ntp master

Configures the device as an authoritative NTP server.

You can specify a different stratum level from which NTP clients get their time synchronized. The range is from 1 to 15.

Step 3

(Optional) show running-config ntp

Example:

switch(config)# show running-config ntp
(Optional)

Displays the NTP configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring an NTP Server and Peer

You can configure an NTP server and peer.

Before you begin

Make sure you know the IP address or Domain Name System (DNS) names of your NTP server and its peers.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp server {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]
  3. [no] ntp peer {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]
  4. (Optional) show ntp peers
  5. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp server {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]

Example:

switch(config)# ntp server 192.0.2.10

Forms an association with a server.

Use the key keyword to configure a key to be used while communicating with the NTP server. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a server. The range for the max-poll and min-poll arguments is from 4 to 16 (configured as powers of 2, so effectively 16 to 65536 seconds), and the default values are 6 and 4, respectively (maxpoll default = 64 seconds, minpoll default = 16 seconds).

Use the prefer keyword to make this server the preferred NTP server for the device.

Use the use-vrf keyword to configure the NTP server to communicate over the specified VRF. The vrf-name argument can be default , management , or any case-sensitive, alphanumeric string up to 32 characters.

Note

 

If you configure a key to be used while communicating with the NTP server, make sure that the key exists as a trusted key on the device.

Step 3

[no] ntp peer {ip-address | ipv6-address | dns-name} [key key-id] [maxpoll max-poll] [minpoll min-poll] [prefer] [use-vrf vrf-name]

Example:

switch(config)# ntp peer 2001:0db8::4101

Forms an association with a peer. You can specify multiple peer associations.

Use the key keyword to configure a key to be used while communicating with the NTP peer. The range for the key-id argument is from 1 to 65535.

Use the maxpoll and minpoll keywords to configure the maximum and minimum intervals in which to poll a server. The range for the max-poll and min-poll arguments is from 4 to 17 (configured as powers of 2, so effectively 16 to 131072 seconds), and the default values are 6 and 4, respectively (maxpoll default = 64 seconds, minpoll default = 16 seconds).

Use the prefer keyword to make this peer the preferred NTP peer for the device.

Use the use-vrf keyword to configure the NTP peer to communicate over the specified VRF. The vrf-name argument can be default , management , or any case-sensitive, alphanumeric string up to 32 characters.

Step 4

(Optional) show ntp peers

Example:

switch(config)# show ntp peers
(Optional)

Displays the configured server and peers.

Note

 

A domain name is resolved only when you have a DNS server configured.

When DNS/Name Server resolves both IPv4 and IPv6, IPv6 Address is preferred by NX-OS.

Step 5

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
(Optional)

Copies the running configuration to the startup configuration.

Configuring NTP Authentication

You can configure the device to authenticate the time sources to which the local clock is synchronized. When you enable NTP authentication, the device synchronizes to a time source only if the source carries one of the authentication keys specified by the ntp trusted-key command. The device drops any packets that fail the authentication check and prevents them from updating the local clock. NTP authentication is disabled by default.

Before you begin

Make sure that you configured the NTP server with the authentication keys that you plan to specify in this procedure.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp authentication-key number {md5 | aes128cmac} password string encryption-type
  3. ntp server ip-address key key-id
  4. (Optional) show ntp authentication-keys
  5. [no] ntp trusted-key number
  6. (Optional) show ntp trusted-keys
  7. [no] ntp authenticate
  8. (Optional) show ntp authentication-status
  9. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp authentication-key number {md5 | aes128cmac} password string encryption-type

Example:

switch(config)# ntp authentication-key 42 md5 aNiceKey

switch(config)# ntp authentication-key 21 md5
JDYk3pp/Fuv0zWyVSRhS6EDERSSsp1uA7s57dvdsx
g74ndf02lEI9dF6WX6Z78/5R8qPmSRRrDUDtCcUlZ
XDUrf0ErodS3ikPQA= 6
switch(config)# ntp authentication-key
12 aes128cmac
JDYkzj4NojJdSkQPvBhFvAO9xCSVwj2iRGvShNSg
ER4JwMBMtUEibfqkscgZ4+/iTdDmeCRW9SGWLxKb
3Xk5g8pz4bR7Iu1a7QA= 6

Defines the authentication keys. The range for authentication keys is from 1 to 65535.

The device does not synchronize to a time source unless the source has one of these authentication keys and the key number is specified by the ntp trusted-key number command.

You can choose either md5 or aes128cmac authentication scheme.

If the user is using the Type-6 key generated from the same primary (master) key, then the device does not synchronize to a time source until the user enables feature password encryption aes.

For Type-0 and Type-7 encryption types, the maximum length is 32 characters. Until Release 10.3(3)F, it was 15 (alphanumeric) characters. For the Type-6 encryption type, the maximum limit is 128 characters.

Step 3

ntp server ip-address key key-id

Example:

switch(config)# ntp server 192.0.2.1 key 1001

Forms an association with a server.

Use the key keyword to configure a key to be used while communicating with the NTP server. The range for the key-id argument is from 1 to 65535.

To require authentication, the key keyword must be used. Any ntp server or ntp peer commands that do not specify the key keyword will continue to operate without authentication.

Step 4

(Optional) show ntp authentication-keys

Example:

switch(config)# show ntp authentication-keys
(Optional)

Displays the configured NTP authentication keys.

Step 5

[no] ntp trusted-key number

Example:

switch(config)# ntp trusted-key 42

Specifies one or more keys (defined in Step 2) that an unconfigured remote symmetric, broadcast, and multicast time source must provide in its NTP packets in order for the device to synchronize to it. The range for trusted keys is from 1 to 65535.

This command provides protection against accidentally synchronizing the device to a time source that is not trusted.

Step 6

(Optional) show ntp trusted-keys

Example:

switch(config)# show ntp trusted-keys
(Optional)

Displays the configured NTP trusted keys.

Step 7

[no] ntp authenticate

Example:

switch(config)# ntp authenticate

Enables or disables authentication for ntp passive, ntp broadcast client, and ntp multicast. NTP authentication is disabled by default.

Step 8

(Optional) show ntp authentication-status

Example:

switch(config)# show ntp authentication-status
(Optional)

Displays the status of NTP authentication.

Step 9

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring NTP Access Restrictions

You can control access to NTP services by using access groups. Specifically, you can specify the types of requests that the device allows and the servers from which it accepts responses.

If you do not configure any access groups, NTP access is granted to all devices. If you configure any access groups, NTP access is granted only to the remote device whose source IP address passes the access list criteria.

  • Without the match-all keyword, the packet gets evaluated against the access groups (in the order mentioned below) until it finds a permit. If a permit is not found, the packet is dropped.

  • With match-all keyword, the packet gets evaluated against all the access groups (in the order mentioned below) and the action is taken based on the last successful evaluation (the last access group where an ACL is configured).

  • peer—process client, symmetric active, symmetric passive, serve, control, and private packets(all types)

  • serve—process client, control, and private packets

  • serve-only—process client packets only

  • query-only—process control and private packets only

The access groups are evaluated in the following order:

  1. peer (all packet types)

  2. serve (client, control, and private packets)

  3. serve-only (client packets) or query-only (control and private packets)

ACL processing of serve-only or query-only depends on the NTP packet type.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp access-group match-all | {{peer | serve | serve-only | query-only }access-list-name}
  3. (Optional) show ntp access-groups
  4. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp access-group match-all | {{peer | serve | serve-only | query-only }access-list-name}

Example:

switch(config)# ntp access-group match-all
switch(config)# ntp access-group peer peer-acl
switch(config)# ntp access-group serve serve-acl

Creates or removes an access group to control NTP access and applies a basic IP access list.

ACL processing stops and does not continue to the next access group option if NTP matches a deny ACL rule in a configured peer.

  • The peer keyword enables the device to receive time requests and NTP control queries and to synchronize itself to the servers specified in the access list.

  • The serve keyword enables the device to receive time requests and NTP control queries from the servers specified in the access list but not to synchronize itself to the specified servers.

  • The serve-only keyword enables the device to receive only time requests from servers specified in the access list.

  • The query-only keyword enables the device to receive only NTP control queries from the servers specified in the access list.

  • The match-all keyword enables the access group options to be scanned in the following order, from least restrictive to most restrictive: peer, serve, serve-only, query-only. If the incoming packet does not match the ACL in the peer access group, it goes to the serve access group to be processed. If the packet does not match the ACL in the serve access group, it goes to the serve-only access group, and so on.

    Note

     

    The match-all keyword is available beginning with Cisco NX-OS Release 7.0(3)I6(1) and is supported on Cisco Nexus 9000 Series switches and the Cisco Nexus 3164Q, 31128PQ, 3232C, and 3264Q switches.

  • The access-list-name variable is the name of the NTP access group. The name can be an alphanumeric string up to 64 characters, including special characters.

Step 3

(Optional) show ntp access-groups

Example:

switch(config)# show ntp access-groups
(Optional)

Displays the NTP access group configuration.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
(Optional)

Copies the running configuration to the startup configuration.

Configuring the NTP Source IP Address

NTP sets the source IP address for all NTP packets based on the address of the interface through which the NTP packets are sent. You can configure NTP to use a specific source IP address.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp source ip-address
  3. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp source ip-address

Example:

switch(config)# ntp source 192.0.2.1

Configures the source IP address for all NTP packets. The ip-address can be in IPv4 or IPv6 format.

Step 3

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring the NTP Source Interface

You can configure NTP to use a specific interface.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp source-interface interface
  3. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp source-interface interface

Example:

switch(config)# ntp source-interface ethernet 2/1

Configures the source interface for all NTP packets. Use the ? keyword to display a list of supported interfaces.

Step 3

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
 (Optional)

Copies the running configuration to the startup configuration.

Configuring NTP Logging

You can configure NTP logging in order to generate system logs with significant NTP events. NTP logging is disabled by default.

SUMMARY STEPS

  1. configure terminal
  2. [no] ntp logging
  3. (Optional) show ntp logging-status
  4. (Optional) copy running-config startup-config

DETAILED STEPS

  Command or Action Purpose

Step 1

configure terminal

Example:

switch# configure terminal
switch(config)#

Enters global configuration mode.

Step 2

[no] ntp logging

Example:

switch(config)# ntp logging

Enables or disables system logs to be generated with significant NTP events. NTP logging is disabled by default.

Step 3

(Optional) show ntp logging-status

Example:

switch(config)# show ntp logging-status
(Optional)

Displays the NTP logging configuration status.

Step 4

(Optional) copy running-config startup-config

Example:

switch(config)# copy running-config startup-config
(Optional)

Copies the running configuration to the startup configuration.

Verifying the NTP Configuration

To display the NTP configuration, perform one of the following tasks:

Command Purpose
show ntp access-groups Displays the NTP access group configuration.
show ntp authentication-keys Displays the configured NTP authentication keys.
show ntp authentication-status Displays the status of NTP authentication.
show ntp logging-status Displays the NTP logging status.
show ntp peer-status Displays the status for all NTP servers and peers.
show ntp peers Displays all the NTP peers.
show ntp rts-update Displays the RTS update status.
show ntp source Displays the configured NTP source IP address.
show ntp source-interface Displays the configured NTP source interface.
show ntp statistics {io | local | memory | peer {ipaddr {ipv4-addr | ipv6-addr} | name peer-name}} Displays the NTP statistics.
show ntp trusted-keys Displays the configured NTP trusted keys.
show running-config ntp Displays NTP information.

Use the clear ntp session command to clear the NTP sessions.

Use the clear ntp statistics command to clear the NTP statistics.

Configuration Examples for NTP

This example for md5 shows how to configure the device to synchronize only to time sources that provide authentication key 42 in their NTP packets: 

switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp authentication-key 42 md5 aNiceKey
switch(config)# ntp server 192.0.2.105 key 42
switch(config)# ntp trusted-key 42
switch(config)# ntp authenticate
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

This example for aes128cmac shows how to configure the device to synchronize only to time sources that provide authentication key 12 in their NTP packets: 

switch# configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
switch(config)# ntp authentication-key 12 aes128cmac password 0/6/7
switch(config)# ntp server 192.0.2.105 key 12
switch(config)# ntp trusted-key 12
switch(config)# ntp authenticate
switch(config)# copy running-config startup-config
[########################################] 100%
switch(config)#

This example shows an NTP access group configuration with the following restrictions:

  • Peer restrictions are applied to IP addresses that pass the criteria of the access list named “peer-acl.”

  • Serve restrictions are applied to IP addresses that pass the criteria of the access list named “serve-acl.”

  • Serve-only restrictions are applied to IP addresses that pass the criteria of the access list named “serve-only-acl.”

  • Query-only restrictions are applied to IP addresses that pass the criteria of the access list named “query-only-acl.” 

switch# configure terminal
switch(config)# ntp peer 10.1.1.1
switch(config)# ntp peer 10.2.2.2
switch(config)# ntp peer 10.3.3.3
switch(config)# ntp peer 10.4.4.4
switch(config)# ntp peer 10.5.5.5
switch(config)# ntp peer 10.6.6.6
switch(config)# ntp peer 10.7.7.7
switch(config)# ntp peer 10.8.8.8
switch(config)# ntp access-group peer peer-acl
switch(config)# ntp access-group serve serve-acl
switch(config)# ntp access-group serve-only serve-only-acl
switch(config)# ntp access-group query-only query-only-acl
switch(config)# ip access-list peer-acl
switch(config-acl)# 10 permit ip host 10.1.1.1 any
switch(config-acl)# 20 permit ip host 10.8.8.8 any
switch(config)# ip access-list serve-acl
switch(config-acl)# 10 permit ip host 10.4.4.4 any
switch(config-acl)# 20 permit ip host 10.5.5.5 any
switch(config)# ip access-list serve-only-acl
switch(config-acl)# 10 permit ip host 10.6.6.6 any
switch(config-acl)# 20 permit ip host 10.7.7.7 any
switch(config)# ip access-list query-only-acl
switch(config-acl)# 10 permit ip host 10.2.2.2 any
switch(config-acl)# 20 permit ip host 10.3.3.3 any

Note

When only a single ACL group is applied, then all the packets relevant for other ACL categories are denied and only packets relevant for the configured ACL group is processed, as mentioned in below scenarios:

  • If serve ACL is configured, then only client, control, and private packets are processed and all the other packets are denied.

  • If serve-only ACL is configured, then only client packets are processed and all the other packets are denied.

If more than a single ACL is configured, it follows the order of processing as mentioned in below scenario:

  • If serve and serve-only both are configured for the same IP address without match-all configured, where the IP is permitted in serve-acl and denied in serve-only, the client, control, private packets are permitted for that IP.