Creating a VLAN interface with ip helper-address only.

[network_geek1979]

Folks,

I am looking for a solution where I can create a VLAN with a ip helper-address only. i.e. this VLAN will not have any L3 address associated to it.
The reason is that we do not want to use this switch as the gateway.

Or else can I use a configuration where the VLAN is configured to use a L3, address and a help-address but when the DHCP server hands out the gateway information this is different than the VLAN ip defined on the switch.

Thanks,

N

[Julio E. Moisa]

Hi

I think that will work if PCs and Server are on the same network or probably on the same L3 device. The computer will broadcast a DHCP request in order to find a DHCP server. The helper is useful because it will forward the broadcast request to a DHCP server on different device. 

[Predrag Jovic]

As Julio Moisa wrote - if DHCP server and host are located in the same broadcast domain or the same L3 device there is no need for IP helper address. Yet, on the same L3 device (host in located in VLAN that ends on L3 switch configured as DHCP server) SVI still needs to be configured with IP address (but not with helper address) - so DHCP server can identify proper IP address range to be offered to host.

If DHCP and host are not located in the same broadcast domain:

Interface VLAN IP address must be configured for IP helper to properly function. When broadcast is converted to directed broadcast SVI adds its own IP address as source when IP packet is created, so if IP address on SVI is not present directed broadcast packet will not be created - simply, because there is no IP address to place in source IP address field.

Source IP address of directed broadcast IP packet is also indication of IP address range (DHCP pool) that should be used to assign IP address to host that is requesting IP address form DHCP server. Whatever IP address for default gateway is configured on DHCP server as default gateway - will be configured as default gateway on host. SVI IP address does not have to be default gateway. Inside of DHCP offer packet - option 3 (router) is specified IP address of default gateway.

So, IP address of SVI need to be from the same IP address range as IP address that you want to be assigned to hosts. DHCP server, when DHCP discovery packet is received, checks IP address of sender (SVI) and IP address in DHCPoffer packet will be from the same IP address range as IP address source inside of IP packet received by DHCP server.