DHCP server and ip helper-address issue

[vancampdax]

Question,

By accident I had configured an IP HELPER-ADDRESS on a VLAN interface pointing to a DHCP server with an IP addrees in the same VLAN ( ip subnet ).

Some users had complaints and there were BAD ADDRESS entries in our DHCP server registered.

Can anyone explain to me why this is an issue please ?

My guess is that the the DHCP server receives the DHCPREQUEST from the client via the braodcast request and via the unicast request from the ip helper-address configuration. But does this really interfere with the DHCPACK and DHCPOFFER packets afterwards ?

[Latchum Naidu]

Hi Dax,

First of of all you no need to have ip-helper address configured on the vlan interface where both dhcp and interface is in same subnet.
You need to configure ip-helper address when your dhcp server is in different subnet.

So please remove the ip-helper command to fix your issue.

Please rate all the helpfull posts.
Regards,
Naidu.

[vancampdax]

Naidu,

I do understand how IP helper-address works but I had by accident configured this on an interface where also my DHCP server was connected too.

The ip helper-address configuration is already removed

I want to understand why I had issues with some clients !

[acampbell]

Hi,

As you had a helper on the same subnet as the server, the server will receive 2 incoming discovers from your PCs MAC/NIC.

One will be a normal broadcast along the LAN

The other will be a DHCP forwarded by the L3 (Router Element)

The server will be confused and may even send back DHCPNACK packets

Regards,
Alex.
Please rate useful posts.

[vancampdax]

Alex,

I've not been able to capture the network packets but I can understand if the server would send DHCPNACK requests ( wxhich would be a normal process ).

I just don't understand why so many users suddenly have issues and my DHCP scope is filling up with BAD ADDRESSES.

My assumption is that the client receives 2 valid DHCP responses ( one form the actual DHCP server and another one from the router, acting as DHCP relay agent ) and acknowledges them, but the DHCP process is somewhere corrupted ( either on the DHCP server or the DHCP client ).

I want a technical explanation for this issue :-)

[acampbell]

Dax,

I am assuming that is a Windows DHCP server.

Have a wee look at this link :-

http://technet.microsoft.com/en-us/library/cc780760(v=ws.10).aspx

Go down to the section :-

Preventing Address Conflicts

Lots of info here about BAD ADDRESSES

Regards,
Alex.
Please rate useful posts.

[Leonardo Gonzaga]

Assuming that I have a multi vlan site with a Windows DHCP dishing out IPs. I have the IP helper command on the right int vlans. How should I configure the port where the DHCP windows server is connected?

Regards, 

Leoanardo de Gonzaga