DHCP for vlan

Unanswered Question
Oct 9th, 2007


we have three vlan's 44,45,46 but dhcp is in vlan 44.All the users are getting ip address.Do we need to put command IP HELPER-address for each vlan ???

I have this problem too.
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guruprasadr Tue, 10/09/2007 - 23:19


Yes, ip helper-address is mandatory.

"ip helper-address" - Forwards UDP Broadcasts, including BOOTP, received on a Interface.


Best Regards,

Guru Prasad R

csc010854800 Tue, 10/09/2007 - 23:29


But in my network it's already working without "ip helper-address" command.

All the PC's are getiing correct IP address.

I am not able to understand how it is working.

Config of my switch is attached herewith..

guruprasadr Tue, 10/09/2007 - 23:54


I hope the DHCP Server and VLANs are in the same Broadcast Domain and normally operating in L2 Mode.

In this way, the DHCP Server and VLANs communicate each other, since DHCP is a Broadcast Protocol.

Hope i am Informative.


Best Regards,

Guru Prasad R

csc010854800 Wed, 10/10/2007 - 00:04

Hi ,

Can u plz tell me any command to verify that they are all in same broadcast domain.

bCZ different vlan's means different broadcast domain .i have 3 vlans,so there must be three broadcast domains.

So, how come they be in the same broadcast domain.

paul.matthews Wed, 10/10/2007 - 00:09

Is there anything else routing between the VLANs? I know there is a reference to a router in VLAN 10 - but does it have interfaces elsewhere? Does the DHCP server ackowledge that it is giving out the addresses? Could there be other DHCP servers? Does the server itself have interfaces in the VLANs?

csc010854800 Wed, 10/10/2007 - 00:32


what they have done is,they put one interface of dhcp in one vlan 44 and second interface of dhcp in other vlan 45 .So, users from respective vlans are getting ip addresses.

Can we do this , like putting DHCP in vlan 44 and issue "ip helper-address" on every vlan.Bcz that will be a good configuration...

guruprasadr Wed, 10/10/2007 - 01:04

HI, [Pls Rate if HELPS]

"DHCP-RELAY" concept will meet your requirement.

Normlly, Relay agent is any Host that forwards dhcp packets between the DHCP Sever and CLients.

DHCP Relay can be used when the Server and Client are not in same Physical Subnet.

DHCP Agent use the support feature of Un-numbered Interface. With this Un-numbered Interface the relay agent automatically adds the static-route to the destination.

RFC for DHCP Relay Agent:


Sample Eg:

interface ethernet 0

ip address


interface serial 0

ip unnumbered ethernet 0

ip helper-address

In the above example, the DHCP relay receives a DHCP request on serial 0 and sends the request to the DHCP server located at IP helper-address When it sends the DHCP request, the DHCP relay uses the IP address of Ethernet 0 for identification purposes and maintains this information for the life of the assigned IP address. This ensures that DHCP packets are sent to the proper DHCP server ( and are returned to the client originator located at the other end of the unnumbered interface (serial 0).


Best Regards,

Guru Prasad R

paul.matthews Wed, 10/10/2007 - 03:48

Having the DHCP server single homed (ie just on one VLAN) is tider, generally safer and scales better than an interface in each vlan.

As has been suggested, ip helper address on all other VLANs will forward packets from those VLANs to the DHCP server.


Mohamed Sobair Wed, 10/10/2007 - 04:24

Hi Guys,

There is a Situations that u can forward UDP broadcast through an IP Helper-address command to a directed-broadcast address on other segment and enable specific interface to forward drected broadcast.


Int s0/1

ip add

ip directed-broadcast

ip helper-address

Does this suffice in this Scenario?


Mohamed Sobair

csc010854800 Wed, 10/10/2007 - 04:28

Hi ,

Can we use this command "ip directed-broadcast" for broadcast traffic between vlan.bcz i have my dhcp in one vlan 44.and users in vlan 44 and vlan 45.

guruprasadr Wed, 10/10/2007 - 04:50

HI, [Pls Rate if HELPS]

"YES" you can go ahead and use this command.

This will allow dhcp server to receive the Broadcast at Layer 2.

Hope I am Informative.


Best Regards,

Guru Prasad R


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