Placement of IP Helper Address

[jeff_turl]

I am just trying to get me head around where i should place IP helper addresses, here is my setup:

Switch1 has the following two VLANs - VLAN50 DATA, VLAN100 Voice.  Interface Gi1/0/1 is a routed port, with address 172.25.200.201/30

default gateway is routed over GI 1/0/1 to the central office.

At the central office, int GI 5/24 is configured with address 172.25.200.202/30 and connects to the other end of the 10mbps EAD circuit.

At the CO, the DHCP server is on vlan40 int GI 3/17

So my question is on which VLAN/Interface do i need to place the helper address?  At the moment, i seem to have it on VLAN50, 100, GI 1/0/1 on Switch1 and also GI 5/24 at the CO.  All addresses point to the same DHCP Server.

Regards

Jeff

[Giuseppe Larosa]

Hello Jeff,

the ip helper address is needed only on the client facing interfaces either SVIs on multilayer switches or routed interface or vlan subinterface on routers.

it is not needed on the link between the branch office and the central office if that link uses statically assigned ip addresses.

the reason is that with ip helper function the router will act as a relay agent for bootp messages:

it changes destination address from broadcast 255.255.255.255 to server unicast ip address making the DHCP request routable across multiple router hops

the GW field internally is populated with the address of the interface that received the original DHCP request so that the the DHCP server can understand from what address pool to pick up an address

the source address is also change to a router's address.

routers on the path to the DHCP server don't need the configuration of the ip helper address on transit links as all the job is done by the first hop router as explained above.

Hope to help

Giuseppe

[Reza Sharifi]

The helper address should be in the SVI for each vlan.  So, switch1 has 2 SVIs (one for vlan 50 and one for vlan 100), that is where you add the IP helper address, and as long as you have connectivity to vlan 40, where the DHCP server resides, then you should be fine.

HTH

[jeff_turl]

Thanks for your replies, so by the sounds of it i can do away with the helper on GI 1/0/1 on the branch switch and 5/24 at the CO.  So it is only needed on the interface closest to the DHCP client.  Giuseppe, that makes sense about changing it from a broadcast to a unitcast, hence why the helper is only needed on the interface/vlan closest to the client.

Thanks

Jeff

[Ven Taylor]

Just remember that a router, by design, blocks broadcasts.  A helper address just tells the router it's okay to forward those brodcasts, but just to the ip address listed as the helper address.

The typical deployment of a helper address is a dhcp server on a different subnet.

The router uses its routing table to determine where to forward the broadcast traffic and needs no other help.

Ven