NTP is not synchronising

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Sep 14th, 2007
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Hi, I have set up and Unix NTP server to synchronise with internet time servers and put the Unix server as ntp server on cisco boxes.

Cisco devices are in subnet

NTP server is in subnet

At the moment cisco boxes are not synching the time with the NTP server. However if I bring the NTP server in to the same subnet as cisco devices it is working fine. I have add both networks in the ntp.conf file



If someone can put me in the right direction to troubleshoot this I would really appreciate it.

Thanks in advance,


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a.alekseev Fri, 09/14/2007 - 07:13
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Can they ping each other?

dlwanuradha Fri, 09/14/2007 - 07:50
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Yeah, they can ping each other. Also the ntp box is directly connected to the access switch. Same management VLAN, even then, they can not synch the NTP.

Richard Burts Fri, 09/14/2007 - 11:07
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I am puzzled at the description of the problem. You say that with current addresses NTP is not syncing with the server but if you move the server into the subnet of the Cisco that they do sync. But the addresses given with the given mask are already in the same subnet (10.20.32 and 10.20.33 are in the same subnet if the mask is

When you say that the ntp server is on the same access switch, is it in the same VLAN as the Cisco? When it works are you moving it into the same VLAN as the Cisco? If they are in the same logical IP subnet but not in the same VLAN that would be a problem that would prevent NTP sync.



dlwanuradha Sat, 09/15/2007 - 13:04
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Hi Rick,

Thanks for the reply. Sorry about the confusion, I have made a mistake in my previous description.

Cisco devices were synching under following settings.

NPT subnet : /22

Cisco Devices were on : and

NTP was on VLAN 7,

Cisco devices management VLAN was 2.

After testing everything is working fine, I wanted to move the NTP server in to the same subnet as Cisco devices and move it in to VLAN 2.

So I configured the access switch port which is connected to the NTP to be on VLAN2 and gave a static IP to the NTP server

With these settings, I can ping from and to NTP server and cisco devices however devices are not synching with the NTP server. But when I move the NTP server back to subnet and move back to VLAN 7 everything is working fine. So I?m pretty confident there?s nothing wrong with the NTP server end.



Edison Ortiz Sat, 09/15/2007 - 17:17
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In a multilayer device, a packet may be sourcing from a different interface that may not be reachable by the destination.

To ensure the packet is being sourced by the correct interface, it's often recommended to have the 'ntp source [interface]' command as part of the ntp configuration.

I suggest that you deploy such feature and use the interface you pinged from the NTP server as the source.


dlwanuradha Mon, 09/17/2007 - 00:45
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I have already configured the "ntp source interface as vlan 2" (same VLAN as the NTP server). Also I tried using a extended ping using vlan 2 as the source interface and ping works.

lgijssel Mon, 09/17/2007 - 01:12
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When the time is differing too much, you need to reload the devices to restart the synchronization process.

If everything else is OK, it will probably work after that.



dlwanuradha Mon, 09/17/2007 - 01:29
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Hi Leo,

Thanks for the input, I have change the clcok settings manually, and when the NTP was on 10.20.8.x range it synchronised all my cisco devices. So time settings are up to date at the moment though it's not associating any more since I moved the NTP in to the new subnet.




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