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New Member

DHCP Server conflicts

Hi everybody.

Its possible that we can have 2 DHCP Servers in a single network? how is the method to avoid conflicts? when the leased time expires could be any kind of problem? how can I avoid it?

I found the following error in a LAN switch in our network. Why could had occurred? how can I prevent that?

Sep 24 08:38:28.249 COL: %DHCPD-4-PING_CONFLICT: DHCP address conflict: 
server pinged 172.22.3.125.

thanks

3 REPLIES
Cisco Employee

Re: DHCP Server conflicts

Hello,

While having 2 DHCP servers on a same network is possible, I would say that the most probable cause of the address conflict is simply that the IP address 172.22.3.125 was configured statically on some device.

It is difficult to prevent these situations from occuring. Any user having its own PDA, laptop or computer can configure its IP address statically and possibly create an address conflct. That is why both DHCP client and DHCP server try to ascertain whether the IP address being assigned is currently free. The DHCP server does it by pinging the IP address before assigning it, and the client performs an ARP query on this IP address after it has been assigned to it. Both these methods, especially the ARP method used on the client, should be able to detect the conflicts.

There are certain mechanisms on Catalyst switches that can prevent users from attaching illegal DHCP servers to the network, or using statically configured addresses instead of acquiring the IP configuration via DHCP. Illegal DHCP servers can be prevented using the DHCP Snooping feature while the clients' address verification can be performed using the IP Source Guard. However, deploying these mechanisms requires a degree of planning.

Best regards,

Peter

New Member

Re: DHCP Server conflicts

Hi Peter. The devices we are using as DHCP are cisco switches 6500. the problem is that the messages are appearing with a lot of more addresess.

Sep 24 08:38:28.249 COL: %DHCPD-4-PING_CONFLICT: DHCP address conflict: 
server pinged 172.22.3.125

thanks for your help

Purple

Re: DHCP Server conflicts

Hi,

You probably have overlapping scopes on your DHCP servers or a rogue dhcp server handing out the same addresses as yours.

Can you post the output of show ip dhcp pool "name_of_pool" on both switches.

Regards.

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