NTP DNS Lookup - Fails at Boot and then removed.

[Rob Craig]

I am hoping there is a quick solution to my query that I am not seeing in my own research.

IE-2000-8TC-B, devices running code 15.0(2)EA1

Configuration uses DNS for NTP and the domain name and 2 name-servers configured:

ntp server <DOMAINNAME.com>
ip domain-name <NAME.com>
ip name-server X.X.X.X
ip name-server X.X.X.Y

After a reload NTP attempts to resolve via DNS, I suspect before the uplink interfaces are fully up, therefore,  the name-servers are not reachable, the resolution fails and the configuration is removed from the running-config and does not attempt to retry connecting.

I can connect and enter the command ntp server <DOMAINNAME.com> and it resolves correctly and everything is fine.

I considered KRON jobs and other methods but it seems like I am over analysing something that potentially has a quick fix which I have missed.

Anyone encountered this or have a configuration suggestion?

Thanks in advance

[Philip D'Ath]

I'm not familiar with that platform, but the general syntax for IOS devices running 15.x code to do recurring DNS lookups (as opposed to a single lookup) for NTP is:

ntp server ip 1.pool.ntp.org
ntp server ip 0.pool.ntp.org

[Rob Craig]

Hi Phil, thanks for responding.

If they both fail (due to startup and a data path not being available) do you think this will loop so they would try again? Fear I would be in the same predicament. That being no NTP synchronised with the relevant NTP configuration present in the startup but not the running configuration file. When using a static IP address the issue doesn't occur and the line makes it from the startup into the running with NTP synchronised.

[Philip D'Ath]

On my kit - yes they keep looping.

The "ip" command causes a DNS lookup everytime it is used, do you can have NTP servers on dynamic IP addresses and it still works.

[Rob Craig]

Thanks , will check with the ip in and see we how we go. 

[Rob Craig]

Hi Philip,

I tried using the "ip" keyword, both with our internal NTP and the two you referenced. Issue persisted. I have attached some captures showing the commands in the file structures for: Running-Config, Startup-Config and Config.Text. The device is then reloaded and all of the NTP DNS entries are removed from all of the configurations. Strange to say the least.

Attempted a couple of things including.

NTP Source VLAN1.

NTP Source <uplink interface>

Clear NTP Drift

Different platform, different code: IE-2000-4TS-G-B on 15.0(1r)EY1

Nothing makes a difference, NTP information is always persistent if I use a straight IP.

I noticed that in the logging buffer from start up: NTP Core (INFO): keys initilized. Occurs before the interfaces are up, but unsure yet if this is related.

Any other thoughts? Thanks in advance.

[Philip D'Ath]

I think it is highly likely you have run into a software bug.  Is their newer firmware for these switches?

[Rob Craig]

I'll try a later version of code on my test box.. can't see any registered BUG-IDs.

Thanks for your time Philip. Much appreciated. Will let you know if I reach an outcome

[Kevin Pond]

Did you ever get a resolution on this. It appears I have the same issue on our 3850 switches running version 03.06.04.E software and on version 15.2(2)E4 on our 2960's. Both IOS are the recommended versions from Cisco.

[Rob Craig]

Sorry Kevin, we had no joy and reverted to an IP implementation.

Good luck