dhcp accross vlans

[carl_townshend]

Hi all

I have come up with a design for one of my offices, I am going to have 1 vlan per floor, there are 3 floors with about 50 machines on each, So what I have done id subnet a class c address off to a /26 for each floor. I will have 1 dhcp server on site. First of all , Is this a good design, and secondly how would I get my dhcp server to issue the subnetted class C addresses to these vlans

thanks

Carlos

[ankurbhasin]

Hi Carlos,

Yes you can do that and it is a good design.

Now to assign each vlan withs its subnet ip range you have to create 2 different dhcp pools.

Can you config will you be configuring cisco layer 3 switch or router as DHCP server or some external dhcp server like windowns etc etc and which device you will be using for intervaln routing between 3 vlans , router or layer 3 switch?

Ankur

[balajitvk]

Hi Carl,

As far the subnet allocation it is correct.

To make your dhcp server to issue the subnetted class C, configure three ip pools with the corresponding gateway address, so that it will start allocating ips from corresponding pools.

You have to give helper-address command in those vlan interfaces with your dhcp ip address to send the dhcp quries to dhcp server.

Rgs

[carl_townshend]

thanks guys

I have 2 options, either to use the router, or I will be using a windows server, my boss will probably go with the server !! I will be using a router for the vlan routing

Can someone tell me what the config should be on the router and dhcp for these assignments ?

Is this a best practice setup guys ?

[amit-singh]

Hi Carl,

Yes that the best practice that is generally used, a dedicated windows server with DHCP funcationality.

Once you are done with the DHCP pool configuration and DHCP settings on the server, you just need to use command " ip helper-address " under the router interface from where the DHCP request will be originated to reach the DHCP server in diff vlan.

For example, if your DHCP server is in Vlan 10 and you have vlan 20 and 30 also and the clients in vlan 20,30 wants to get an Ip from DHCP, do the following config under the vlan interface :

conf t

interface vlan 10

ip add x.x.x.x

interface vlan 20

ip add y.y.y.y

ip helper-address x.x.x.* <-- IP of the DHCP server

interface vlan 30

ip add z.z.z.z

ip helper-address x.x.x.* <-- IP of the DHCP server.

HTH, please rate if it does.

-amit singh

[carl_townshend]

thankyou, What pools will I need to assign on the dhcp server, will they just be the same with the subnet masks I have issued !!

thanks

[amit-singh]

They will be diff ofcourse. It depends on your IP addressing scheme that you want to use,For ex.. you might want to create 192.168.10.0/26 for vlan 10, 192.168.20.0/26 for vlan 20 and 192.168.30.0/26 for vlan 30.

HTH, please rate if it does.

-amit singh

[carl_townshend]

when using this helper address, will that particular vlan with ip helper configured seek across all vlans to find the dhcp server ?

[amit-singh]

No, When the DHCP broadcast request will come to the vlan interface for the DHCP request, the vlan interface with ip helper configured makes a unicast request to the DHCP server which is conifgued in the ip helper command. The request will directly to go to the DHCP server and a reply back will be forwarded to the requesting client.

HTH, please rate if it does.

-amit singh