DHCP FAILURE

[Benkip]

Dear Experts,

I am experiencing below DHCP error on some clients will connecting on Wi-Fi. Anyone with a solution to the issue.

"Client made a request to the DHCP server, but the DHCP server rejected the client's request.server_ip='192.168.X.X' vlan_id='40' details='dhcp_nack' radio='1' vap='0' channel='44' rssi='22'"

[marce1000]

         - Check this discussion https://www.reddit.com/r/meraki/comments/11mtrc8/sporadic_dhcp_issues/

 M.

[Benkip]

Hi Marce, Thanks for the insight. I actually have a small number of devices (less than 200). On the network, we have a guest ssid with dhcp functionality that runs on the core switches. I will give it a try but I doubt if that will be the cause of the issue. When I connect device using cable on the same Vlan as the dhcp server, the devices connect immediately.

[cmr]

Are you setting DHCP relay on a Meraki switch?  There are a number of bugs and fixes about that.  What switch model(s) and firmware are you using.

[Benkip]

Yes, using dhcp relay.

Switch model - ms210-24p

Firmware Version: MS 15.22

Will really appreciate the fixes.

[cmr]

The bug was for MS120 switches, so I think this might be something else, sorry.

[Benkip]

May be it can help, actually access switch that connect to the APs are MS120 and all running 15.22. The coreswitch is MS210.

[cmr]

It looks like you have the fixed version...

[Benkip]

Noted and thanks Cmr

[BlakeRichardson]

 Is your DHCP lease pool large enough? Private MAC if enabled on devices can exhaust a DHCP lease pool very quickly if you have a few users and long DHCP lease times. 

[Benkip]

Thanks Blake. DHCP lease pool is large. I am using less than 50%. I did reduce the DHCP lease time to 8 hours. I am still getting logs with the same error. Private MAC is disabled as well.

[Rimccart]

Hi Benkip, 

The logs for the DHCP server rejection is curious. What are the DHCP server logs indicating when the wireless client is attempting to connect to the network? If a capture is taken at the DHCP server to the DHCP server switch port is the NAK originating from the server? Did the update to MS15.22 alleviate the believed unexpected behavior that was being hit?

[Benkip]

Hi Rimccart,

We are still experiencing the same issue.

We had two dhcp servers on active active mode initially enabled for the affected SSID. From packet capture on the ports dhcp servers, ACK messages were only being received from on server. We deactivated the active active mode on dhcp and relay to the only one dhcp server were getting ACK messages from. 

Firmware update did not also help.

Thanks.

[Rimccart]

Hi Benkip, 

It sounds like this is only impacting one SSID, correct? 

If this is only impacting a particular SSID are there any settings on this SSID that drastically differ from the others? This could be the SSID configurations or the RF profile settings as well if the bitrates are set per SSID. 

When we see this DHCP failure log in Meraki have we examined the impacted client's timeline as well?

Were there any additional events just before this DHCP failure, such as roaming or disconnection events?

[Benkip]

Hi Rimccart, The NAK messages disappeared today after changing the dhcp port from trunk to access. I had configured the port that connect the dhcp server to trunk with native vlan 1. I think the dhcp server was responding to numerous dhcp requests from different vlans while on trunk hence generating the NAK messages. Happy this has resolved the issue. Thank you very much for your input.