106
Performing Switch Administration
Information About Performing Switch Administration
NTP avoids synchronizing to a device whose time might not be accurate by never synchronizing to a device that is not
synchronized. NTP also compares the time reported by several devices and does not synchronize to a device whose time
is significantly different than the others, even if its stratum is lower.
The communications between devices running NTP (known as
associations
) are usually statically configured; each device
is given the IP address of all devices with which it should form associations. Accurate timekeeping is possible by
exchanging NTP messages between each pair of devices 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
device can simply be configured to send or receive broadcast messages. However, in that case, information flow is
one-way only.
The time kept on a device is a critical resource; you should use the security features of NTP to avoid the accidental or
malicious setting of an incorrect time. Two mechanisms are available: an access list-based restriction scheme and an
encrypted authentication mechanism.
Cisco’s implementation of NTP does not support stratum 1 service; it is not possible to connect to a radio or atomic clock.
We recommend that the time service for your network be derived from the public NTP servers available on the IP Internet.
shows a typical network example using NTP. Switch A is the NTP master, with Switches B, C, and
D configured in NTP server mode, in server association with Switch A. Switch E is configured as an NTP peer to the
upstream and downstream switches, Switch B and Switch F.
Figure 14
Typical NTP Network Configuration
If the network is isolated from the Internet, Cisco’s implementation of NTP allows a device to act as if it is synchronized
through NTP, when in fact it has learned the time by using other means. Other devices then synchronize to that device
through NTP.
Switch F
Switch A
Workstations
Workstations
Local
workgroup
servers
101349
Switch B
Switch E
Switch C
Switch D
Содержание IE 4000
Страница 12: ...8 Configuration Overview Default Settings After Initial Switch Configuration ...
Страница 52: ...48 Configuring Interfaces Monitoring and Maintaining the Interfaces ...
Страница 108: ...104 Configuring Switch Clusters Additional References ...
Страница 128: ...124 Performing Switch Administration Additional References ...
Страница 130: ...126 Configuring PTP ...
Страница 140: ...136 Configuring CIP Additional References ...
Страница 146: ...142 Configuring SDM Templates Configuration Examples for Configuring SDM Templates ...
Страница 192: ...188 Configuring Switch Based Authentication Additional References ...
Страница 244: ...240 Configuring IEEE 802 1x Port Based Authentication Additional References ...
Страница 274: ...270 Configuring SGT Exchange Protocol over TCP SXP and Layer 3 Transport Configuring Cisco TrustSec Caching ...
Страница 298: ...294 Configuring VLANs Additional References ...
Страница 336: ...332 Configuring STP Additional References ...
Страница 408: ...404 Configuring DHCP Additional References ...
Страница 450: ...446 Configuring IGMP Snooping and MVR Additional References ...
Страница 490: ...486 Configuring SPAN and RSPAN Additional References ...
Страница 502: ...498 Configuring Layer 2 NAT ...
Страница 559: ...555 Configuring Network Security with ACLs How to Configure Network Security with ACLs Creating a Numbered Extended ACL ...
Страница 770: ...766 Configuring IPv6 MLD Snooping Related Documents ...
Страница 930: ...926 Configuring IP Unicast Routing Related Documents ...
Страница 956: ...952 Configuring IPv6 Unicast Routing Configuring IPv6 network 2010 AB8 2 48 network 2010 AB8 3 48 exit address family ...
Страница 976: ...972 Configuring Cisco IOS IP SLAs Operations Additional References ...
Страница 978: ...974 Dying Gasp ...
Страница 990: ...986 Configuring Enhanced Object Tracking Monitoring Enhanced Object Tracking ...
Страница 994: ...990 Configuring MODBUS TCP Displaying MODBUS TCP Information ...
Страница 996: ...992 Ethernet CFM ...
Страница 1030: ...1026 Working with the Cisco IOS File System Configuration Files and Software Images Working with Software Images ...
Страница 1066: ...1062 Using an SD Card SD Card Alarms ...