Chapter 9 - Device Management Protocols - NTP
Routers and switches use the default stratum level of ____ for their internal reference clock based on the default setting.
8
Flip to see the output of the commands show ntp status and show ntp associations on a router acting as an NTP server.
In the show ntp associations command output at the bottom, note that loopback address 127.127.1.1, along with a reference clock value of ".LOCL."
Flip to see a network topology that illustrates the idea of NTP stratum level.
The NTP primary server (R3) has a stratum of 2. R2, which references R3, adds 1 so it has a stratum of 3. R1 uses R2 as its NTP server, so R1 adds 1 to have a stratum of 4.
What is the range of stratum level you can configure on a Cisco router/swtich ?
1 through 15
An NTP server uses the loopback address ____ for the ip address of its reference clock to indicate that it gets its clock internally.
127.127.1.1
All NTP clients and servers in a network use the same stratum level. True or False ?
False. An NTP server uses its internal hardware or external reference clock sets its own stratum level. Then, an NTP client adds 1 to the stratum level it learns from its NTP server, so that the stratum level increases the more hops away from the original clock source.
All devices in an enterprise should reference external NTP servers. True or False ?
False. Only a few enterprise devices should refer to those external NTP servers and then act as both NTP client and server. The majority of the devices in the enterprise would act as NTP clients.
What is the benefit of using the increasing stratum levels ?
It allows devices to refer to several NTP servers and then use time information from the best NTP server, best being the server with the lowest stratum level.
What does the following global configuration command do ? clock summer-time EDT recurring
It configures the system to switch automatically to summer time (daylight saving time). The 'EDT' parameter sets the timezone to Eastern Daylight Saving Time. The 'recurring' keyword tells the router to spring forward an hour and fall back an hour automatically over the years.
What does the following global configuration command do ? ntp source loopback 0
It tells a router that is acting as an NTP server to use the IP address of loopback interface 0 as the source address when sending NTP packets.
What are the other two terms for NTP server mode and NTP client/server mode ?
NTP server mode ---> NTP primary server NTP client/server mode ---> NTP secondary server
What are the 2 modes of NTP ?
NTP server mode — the device acts only as an NTP server, and not as an NTP client. The device gets its time information from the internal clock on the device. NTP client/server mode — the device acts as both client and server. First, it acts as an NTP client, to synchronize time with a server. Once synchronized, the device can then act as an NTP server, to supply time to other NTP clients.
What does the following EXEC command do ? clock set 20:52:49 21 October 2015
The clock set EXEC command sets the time, day of the month, month, and year to 20:52:49, 21 of October 2015
What does the following global configuration command do ? clock timezone EST -5
The parameter "EST" sets the the timezone of the device to US Eastern Standard Time (EST). The "-5" parameter means that this device is 5 hours behind Universal Time Coordinated (UTC) .
What is the NTP Stratum Level ?
The stratum level is a number that shows the perceived accuracy of an NTP client's reference clock. The lower the stratum level, the more accurate the reference clock is considered to be.
How does devices that act solely as an NTP server get their time ?
They can get their time from either internal device hardware or from some external clock using mechanisms other than NTP.
Flip to see the output of the command show ntp associations
This command lists all the NTP servers that the local device can attempt to use, with status information about the association between the local device (client) and the various NTP servers. Beginning with R1, note that it has one association (that is, relationship with an NTP server), based on the one ntp server 172.16.2.2 configuration command on R1. The * means that R1 has successfully contacted the server. You will see similar data from the same command output taken from router R2.
It is a good practice to set the correct date and timezone on a device before enabling NTP client function on it and synchronizing its time-of-day clock with an NTP server. True or False?
True
For good design, the enterprise NTP configuration ought to refer to at least _____ external NTP servers for redundancy. 1. One 2. Two
Two
What is the command to set the device's time zone to Singapore time?
clock timezone SGT 8
Besides using a networking device as the reference clock (NTP server) for the enterprise, what are the other options you can use?
you can reference better time sources in the Internet or purchase a purpose-built NTP server that has better clocking hardware.
What is the one potential issue of using external NTP servers ?
if routers configured to use external NTP servers no longer hear NTP messages from the NTP servers in the Internet they will lose their only reference clock. After losing their reference clock, they could no longer be useful NTP servers to the rest of the enterprise.
Flip to see the output of the command show ntp status
it lists a status of synchronized, which confirms the NTP client has completed the process of changing its time to match the server's time. Any router acting as an NTP client will list "unsynchronized" in that first line until the NTP synchronization process completes with at least one server. It also confirms the IP address of the server—this device's reference clock—with the IP address configured as 172.16.2.2.
What command can you use to set the stratum level of a router/swtich acting as an NTP server ?
ntp master <stratum_level>
What is the show command to display the time-of-day clock on a Cisco device ?
show clock
