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So currently we have R1 and R2 as NTP servers at Stratum 1.

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We have R3 as they client to R1 and R4 as they client over to R2.

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So if we had R1 and R2 synchronizing with some external atomic clocks or time

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servers out on the

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internet that were Stratum 0, what happens if one of those is no longer

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available?

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We don't want those key servers to give up regarding time.

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We still want them to have a way of synchronize or if R3 and R4, if they lose

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connectivity,

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for example to R1 and R2.
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for example to R1 and R2.

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So think of a peer as like a backup friend you can go to in the event that the

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time server that

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you are synchronizing with is no longer reachable.

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And that way these two devices can ask of the time of each other if they need

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to and then

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synchronize their time that way.

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So for the demo I'm going to go ahead and specify that R3 is going to be a peer

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with R4

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and R4 is going to appear with R3.

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Now because they're both getting their time from a Stratum 1 NTP server,

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they're going to actually start querying each other and actually believing the

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information from
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information from

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each other until they lose connectivity to their respective time servers at Str

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atum 1.

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And it'd be at that point that they start reaching out and asking for the time

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and

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synchronize the time with their peer.

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Think of it like a fault tolerance or a fallback method for making sure that we

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have the correct

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time even if our primary time servers fail.

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So with that in mind, let's head over to R3 and then R4 and we'll set them up

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as NTP peers.

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So back in the lab, let's go to R3.

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We'll start there and on R3, let's go ahead and do a show debug.
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We'll start there and on R3, let's go ahead and do a show debug.

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And let's do a debug of NTP everything.

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So debug NTP all and I'll do the same thing on R4.

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We'll do a debug of NTP all.

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And let's also verify where we're at as far as NTP by going to R3.

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I'm doing a show NTP status.

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Fantastic.

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We're synchronized with the server at 1111.

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Let's also do a show NTP associations press center.

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Here it shows we are configured to use 1.1.1.1 as our time source and little

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asterisk here at the

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far left means that we're synchronizing with that server.
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far left means that we're synchronizing with that server.

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All right.

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So to configure R4 as an NTP peer, in fact, let me go do a do on debug all and

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then we can look

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at the debug over on R4 if we need to.

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So here we'll do an NTP and instead of specifying server, we're going to go

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ahead and specify

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peer.

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That's this option right here.

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So NTP peer will put in the IP address of the loopback on R4, which is 4.4.4.

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We'll specify the source for our packet address.

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And over there is the IP address on our loopback zero interface.
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And over there is the IP address on our loopback zero interface.

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So we'll put that in and then we can also go ahead and specify things like the

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men

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pull time on put four, which is which is two to the power four, which means 16

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and the max

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pull also at the same value.

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And I'm also going to say burst, even though I think it's on by default and we

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'll press

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center and then we'll do a similar treatment over at R4.

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So we'll go to R4 and on R4, we'll go config T and we'll type in NTP peer and

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put in 3,

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3, 3, 3, source, loopback zero, men timer, four and max timer, four for the lab
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3, 3, 3, source, loopback zero, men timer, four and max timer, four for the lab

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and then the keyword

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of burst.

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So now if we do this show, in fact, let me turn off debugging here as well.

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Do an unall as a show NTP status.

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So notice that we are still, oh, check it out.

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We got it during a decision point.

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So it is not synchronized at the moment.

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Let's give it a moment to go ahead and correct that will do a show NTP status.

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And while we're waiting for the synchronize will do a show NTP associations and

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press center.

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So here, you'll notice there's no asterisk on the left hand side.
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So here, you'll notice there's no asterisk on the left hand side.

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So it knows about two, two, two, two, which is a stratum one server.

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It knows about three, three, three, three as a peer and it's a stratum two

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server.

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So after we give it a few more moments here, it should once again synchronize

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its clock back to two,

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two, two, and then also be aware that three, three, three, three exists as well

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.

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And let's go to our three and over here on our three with debugging not on list

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to a show NTP status,

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press center.

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And that guy is synchronized to the server at 1111.

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And if we do a show NTP associations and press center,
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And if we do a show NTP associations and press center,

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here we have the asterisk next to the 1111 indicating that we are synchronized

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with that NTP server.

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However, we still have our four in the wings in case our primary server goes

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away.

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And we can synchronize with it.

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So let's go back to our four and one of the verify that our four is synced up.

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So let's show NTP status press center and it still says unsynchronized.

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So a couple options here.

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We could just wait, which is perfectly fine.

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If you want to do some debug like debug NTP all press center.

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We could look at the debug messages.
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We could look at the debug messages.

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We could do a show NTP status.

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So we are communicating with R2.

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That's great.

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And at some point in time, we will synchronize once again with our primary NTP

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server,

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the stratum one server, which is part two.

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And another option is let's do a do undibug all as to a do show run include NTP

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.

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Now this part right here is really just to keep us busy while we're waiting for

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NTP to

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synchronize because by itself it will just synchronize.

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But if you're impatient or want to force it, one thing I've discovered over the
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But if you're impatient or want to force it, one thing I've discovered over the

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years is that

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if we take out the configuration, put it back in, that's often very helpful.

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So let's go ahead and do a no.

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And I'm going to take off the NTP server here, no NTP server at 222.

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And then I'll put it back in with the right click and we are back in business.

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Also, I didn't specify the min and max timers.

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So if we wanted to, we could go ahead and do that with a min of four and a max

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of four

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and versus on by default, but if it makes us feel better, we can try to force

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it that way

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and then just give it a little bit of time.
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and then just give it a little bit of time.

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So again, if you did the debug of NTP all press center,

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that would help us identify what's going on that we're actually communicating

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back and forth

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with the respective other NTP devices.

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And once it says that the clock is synchronized, it'll also show up in the show

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NTP status

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that the clock is synchronized to the NTP server over at R2.

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So I'm going to do an debug all and show NTP status.

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Oh, there we go.

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Oh my gosh.

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We just turned off the debug before I got the message about synchronization.

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So we are now synchronized back to R2 and we do a show NTP associations and
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So we are now synchronized back to R2 and we do a show NTP associations and

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press center.

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Here we have the little asterisk indicating we are synchronized with a server

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and the tilde represents we have full communication back and forth with that

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device.

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And again, if we do a show NTP status, it'll confirm for us that we are

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synchronized with our

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server.

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So if you're working with a Cisco device and you're working with an NTP server,

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you'd want to make sure a few things are going your way.

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Number one, are the ports for NTP? Are they open and available?

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Meaning are we allowing UDP port 123?

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So if we went up to R1, for example, or R2,
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So if we went up to R1, for example, or R2,

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and we did a show control plane host open ports and press center,

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you'll notice that UDP port 123 is open in listen mode because we're acting as

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an NTP server.

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So we want to make sure if we have firewalls or anything else in place between

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our NTP clients

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and our NTP servers, we want to make sure that we're allowing those specific

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protocols through

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so NTP can operate.

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And we also want to incorporate some fault tolerance such as NTP peers and

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multiple NTP servers that we can communicate with.

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So if we have a failure of one device in our network, it's not going to ruin
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So if we have a failure of one device in our network, it's not going to ruin

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our time stamps

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across the board.

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So now that we looked at the NTP configuration as well as some confirmation

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that is working,

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in the next video, I'd like to put you, my friend, on a hands-on lab so you can

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get up close and

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personal hands-on practice and build your skills in working with the

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configuration

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and verification of network time protocol.

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So when you're ready, I'll see you in the next video in just a moment.