Dhcp ip helper question

[mahesh18]

                   Hi everyone,

We have two remote sites and they are connected via ospf.

Site 1 has Router 1 and site 2 has router 2.
Router 1  which has dhcp config on router for data and Voice pool.

ip dhcp pool data Site 1

network 10.74.150.0 255.255.255.128

default-router 10.70.150.1

ip dhcp pool data Site 2

network 10.74.151.0 255.255.255.128

default-router 10.70.151.1

Voice  DHCP pool site 1

!

ip dhcp pool Voice  Site 1

network 10.74.52.0 255.255.255.128

default-router 10.74.52.1

!

ip dhcp pool Voice  site 1

network 10.74.52.128 255.255.255.128

default-router 10.74.52.1

Voice DHCP  pool Site 2

ip dhcp pool voice site 2

network 10.74.53.0 255.255.255.128

default-router 10.74.53.1

ip dhcp pool voice site 2

network 10.74.53.128 255.255.255.128

defailt-router 10.74.53.1

data has vlan 2 and voice has vlan 3.

My question is ip helper address command  on router 2 which is at site 2

1>what ip address should i use on ip helper

option 1 r1 ospf  ip address ?

2>where should i apply ip helper address command on vlan 2  and vlan 3 or under R2 ospf interface

thanks

mahesh

[Richard Burts]

Mahesh

You would configure the ip helper-address command on router 2 on the interface where the client request for DHCP is received. And the address to use in the helper-address command is the address of the server that will offer the DHCP address, so it would be the address of router 1.

HTH

Rick

[mahesh18]

Hi Rick,

Thanks for reply.

Here is what i found after doing some testing.

On R2 router if i config ip helper address under R2 wan interface it does not work.

But when i config ip helper address under data vlan 2  ip helper works fine.

Also what ip address to use for ip helper  here is testing results

when i use default router ip address under dhcp pool of data it does not work.

When i use the ospf nei ip address of R1 router   then pc connected to R2 is able to get ip address and its gateway is

default router ip under dhcp pool.

Need to confirm if this is correct way to going forward?

Thanks

MAhesh

[Richard Burts · Accepted solution]

Mahesh

I think that these are the correct behaviors. I said the helper-address should be on the interface where the client request is received and the client request comes in to the vlan interface and not in to the wan interface. And the helper-address should be the address of the router where the DHCP is configured, so the OSPF neighbor address would be the correct address. So your configurations are the correct way to go forward.

HTH

Rick

[mahesh18]

Hi Rick,

Many thanks again.

Regards

MAhesh