05-11-2012 02:59 AM - edited 03-07-2019 06:38 AM
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 ?
05-11-2012 03:42 AM
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.
05-11-2012 04:14 AM
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 !
05-11-2012 04:22 AM
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.
05-11-2012 04:33 AM
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 :-)
05-11-2012 04:54 AM
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.
03-31-2015 02:40 AM
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
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