Jan 19

Cisco 6500 Ignores NTP Requests

I faced an issue with one of our 6500 series switches where 6500 acting as NTP server was not replying to some NTP client which access the switch on a certain port. The clients accessing the switch on a different port were able to sync OK. The version used on the switch is quite quite old but this issue can happen to the new IOS as well.

Cisco Internetwork Operating System Software
IOS ™ s72033_rp Software (s72033_rp-PSV-M), Version 12.2(17d)SXB1,  RELEASE SOFTWARE (fc2)

The problem started when we changed one of the ports connecting our WAN circuit from a switchport to a routed port. Previously this WAN port was part of VLAN and all WAN client use to connect to switch using this port. However, due to certain routing changes, we had to change this port from switch port to a routed port which broke the NTP for those clients and client status changed to unsynchronised. All clients which were accessing the switch internally were ok, as they port they used to access never changed.
I had to go through a long painful process by analysing the traffic on the port, wireshark, debugs and everything I could do. Removed the NTP config from switch and added it back on but it still didnt resolve the issue. Did this to server and clients for dozen time but no effect. On wireshark I could see that the packets from remote clients are getting to switch but switch doesn't reply to them. Finally, found this bug which was applicable to some other IOS versions, tried the fix that was mention in the bug and it resolved the issue.

All you need to do is to use the following for two times on the switch

no ntp
no ntp

and then re-enter the NTP related config back to switch and it should start working for everyone.

The bug ID is   CSCtf03928

However, on Cisco website the effected version doesn't include the version that's on our switch


NTP Working

R1—————(Switch Port)6500(Switch Port)—————R2

NTP Not Working for R2 when

R1————–(Switch Port)6500(Routed Port)—————-R2

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