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Beginner

DHCP server and ip helper-address issue

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 ?

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DHCP server and ip helper-address issue

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.

Beginner

DHCP server and ip helper-address issue

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 !

Advocate

DHCP server and ip helper-address issue

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.

Regards, Alex. Please rate useful posts.
Beginner

DHCP server and ip helper-address issue

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 :-)

Advocate

DHCP server and ip helper-address issue

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.

Regards, Alex. Please rate useful posts.

Assuming that I have a multi

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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