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WiFi/WAP and %DHCPD-4-PING_CONFLICT: DHCP address conflict

Hi all,

The department I work in will be gradually taking over management of WiFi hotspots for a number of large hotels. Today we experienced a situation at one site that we would like to NOT have to deal with again but we're not sure how to address the situation.  Here's the scenario...

The site has ~50 WAPs in a /26 DHCP pool with 62 usable addresses. When the router running the DHCP server reloaded it saw those 50 WAPs using the IPs they'd been assigned and marked those IPs as conflcts. That meant when it came time for the WAPs to release/renew their leases there were only ~10 IPs available in the pool. We got a call from the site saying the whole site was down.  We had to go into the router and manually apply the "clear ip dhcp conflict" command.

What I don't understand is if this is how DHCP works, how does DHCP work at all? I've never heard of this happening before anywhere I've worked and I can't believe it's because DHCP servers have never reloaded. There has to be some workaround or something ... the only thing I can find is a reference to turning off the logging of the conflicts with the command "no ip dhcp conflict logging" however if I'm reading that correctly all it does is stop putting messages in the log file ... that doesn't stop the DHCP server from marking conflicting addresses as unavailable IPs.  If that's the case then that command is of no use to me.

Unfortunately I work the night shift and the place is a ghost town so I don't have access to much equipment to lab things up.  I've been trying to lab it up in GNS3 for the last few hours to replicate the problem and haven't been able to do so, probably because the software is limited in its ability to replicate the hardware we're really using ... so I decided it was time to post a question on here and see if anyone else has run into this problem.

Thanks in advance

2 REPLIES
New Member

Re: WiFi/WAP and %DHCPD-4-PING_CONFLICT: DHCP address conflict

This appears the way Cisco DHCP works. You can read about it in this article: http://blog.ioshints.info/2007/08/dhcp-conflict-logging-true-story.html

You might have success by disabling DHCP ping, but that is not really recommended.

A better solution would be using a third party DHCP solution like infoblox or microsoft DHCP.

Normally when a DHCP server reboots it loses its table of leases. This should not be a problem as the table will be rebuilt over time. Even a DHCP server which uses ping to determine whether the IP address is available will eventually regain this IP address as the client gets a new IP address (and releases the previous one).

WiFi/WAP and %DHCPD-4-PING_CONFLICT: DHCP address conflict

>>When the router running the DHCP server reloaded it

What make/model of router?

After solving the immediate problem, I would also investigate why the router reloads.  The person before me is correct, even after a reload it should not retain a list of used addresses (at least the ones I worked on).  The only way it would keep the IPs is if they are statically binded....I believe, I have only worked on this is labs.

http://www.cisco.com/en/US/docs/ios/12_3t/12_3t11/feature/guide/gtdhcpsm.html

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