IP-Helper Broadcast overwrite native broadcast?

Unanswered Question
Sep 10th, 2009

Hi all, I have a small question. Lets say I have two DHCP servers one in my native subnet and the other in another subnet. I understand that I will have to add an IP-Helper address to my L3 interface for the scope that is outside the native subnet however, once I add that ip-helper address will that overwrite broadcasts within the native subnet? In other words would I also have to add another IP-Helper address for the DHCP server in the native subnet or just keep it at the one helper address for the non-native subnet.



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Edison Ortiz Thu, 09/10/2009 - 16:16

You mean local subnet vs remote subnet?

The router will forward the DHCP request from the client subnet to the DHCP server with the ip helper-address configured but it won't change any local broadcast behavior in that subnet.

If there is another DHCP server in that local subnet, it can provide DHCP services to local clients.

On such scenario, whatever DHCP server replies first - will provide the DHCP lease for the client(s).



yagnesh_tel Thu, 09/10/2009 - 16:18

You don't require to configure 'ip helper address' for DHCP server in the native subnet as this command will not obstruct broadcast within the native subnet.

DHCP broadcast request from client will be received by both servers and client will accept an IP from the first DHCP server that responds.

Richard Burts Fri, 09/11/2009 - 04:21


Perhaps it would help to think about it in terms of how the client sends the broadcast on the local LAN. The server on the local LAN will receive the broadcast directly from the client and there is no opportunity for the router to interfere or overwrite the broadcast on the local LAN.




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