different ip subnets for different vlan

[oddyager]

say i have 2 different vlans on a switch but I want one to pull addresses from 10.1.1.0 and the other from 10.1.2.0 from the same dhcp server (i'll create 2 different scopes on the server itself). how can i tell each vlan to grab from where?
thanks!

 

[spidey07]

tell the router where to forward dhcp requests. Technically it's called a DHCP relay agent.

in cisco terms it's "Ip helper address"

Just setup the scopes on the dhcp server. Then the relay agent will forward the dhcp requests to the server. the relay agent will insert into the dhcp request a field that lets the server know what subnet the request came from.

 

[nweaver]

Originally posted by: spidey07
tell the router where to forward dhcp requests. Technically it's called a DHCP relay agent.

in cisco terms it's "Ip helper address"

Just setup the scopes on the dhcp server. Then the relay agent will forward the dhcp requests to the server. the relay agent will insert into the dhcp request a field that lets the server know what subnet the request came from.

this works really well, btw, I am running 6 or 8 subnets off one DHCP server

 

[spidey07]

Originally posted by: nweaver
this works really well, btw, I am running 6 or 8 subnets off one DHCP server

pssh, I've got about 10000+ on two dhcp servers. Neener.

eitherway, best practice is to have two dhcp servers for all subnets.

 

[RebateMonger]

Originally posted by: spidey07
Just setup the scopes on the dhcp server. Then the relay agent will forward the dhcp requests to the server. the relay agent will insert into the dhcp request a field that lets the server know what subnet the request came from.

I always wondered:

What do you do about any DHCP clients that are on the SAME subnet as the DHCP Server (and there's no router between them and the DHCP Server)? How do THOSE clients get an IP address from the "correct" Scope?

 

[spidey07]

there's no relay agent information in the dhcp request (hasn't passed through a router/relay agent), so the dhcp server assumes it is local and OFFERS an address from the scope of its local NIC.

 

[oddyager]

sorry if this sounds stupid (it probably is) but what do you mean on the router? this is kind of ghetto demo of what i have setup:

several 3550 switches --> 6500 backbone switch --> several firewalls here --> several routers here

so do i add the ip helpder address command on the 6500? i tried and did not see any commands for (I also tried the routers and don't see the command either). I'm running the latest IOS software.

EDIT:

Actually, I do see the command when I'm in interface but I'm not understanding how that works. 🙂 Do I have to add the ip helper address on ALL of my routers?

 

[spidey07]

you only need the IP helpder address on the router interface that defines that subnet. Basically it takes the dhcp broadcast and turns it into a directed datagram to the server. Normal routing delivers it.

so whatever router is the default gateway for this subnet, the ip helper goes there. In your instance it is either the 3550s or the 6500 that have the SVI (vlan interface) for that network.

 

[oddyager]

SPidey, thanks. Is there a command I can run to verify ip helper address?

 

[spidey07]

sh ip int vlan <vlan number>

 

[oddyager]

Awesome thanks. I see how it can be setup using the switch as the default gateway. what if i use a pix as the default gateway though? are the commands the same?

 

[spidey07]

good god man, I ain't gonna spoon feed you everything.

😉

 

[oddyager]

you've gone this far. you must! i'll dig it up on cisco. thanks for your help again.

 

[jlazzaro]

For the PIX, set the IP address of the DHCP server to which the relay agent forwards requests:

dhcprelay server 192.168.1.10 inside

Setup dhcprelay for the name of the interface on which the DHCP relay agent accepts client requests.

dhcprelay enable inside1
dhcprelay enable inside2

Use show dhcprelay to verify

 

[cheesecurd]