Cisco Catalyst 9500 Manual page 18
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Poll-Based NTP Associations
configuration complexity because each device can simply be configured to send or receive broadcast messages.
However, in that case, information flow is one-way only.
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.
System Management Configuration Guide, Cisco IOS XE Fuji 16.8.x (Catalyst 9500 Switches)
4
Administering the Device
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