topic Re: How to use Server based DHCP with ICX switches in ICX Switches

How to use Server based DHCP with ICX switches

r_gent — Fri, 12 Feb 2021 15:33:07 GMT

Hi,

currently trying to get dhcp to work with a few new 7150's we dhcp running on one of our server connected to the primary switch.

We have multiple subnets for various areas. but can figure out how to use the switch as a relay for dhcp.

Re: How to use Server based DHCP with ICX switches

BenBeck — Fri, 12 Feb 2021 15:38:14 GMT

Hey There,

You would add the helper statement under either the routed interface or VE. This is generally done at the default gateway for the respective vlan. An example might look something like this:

conf t

interface ve 10

ip helper-address 1 192.168.20.1

192.168.20.1 would be a DHCP server in a different subnet.

Re: How to use Server based DHCP with ICX switches

paul_mcguire_20 — Fri, 12 Feb 2021 15:41:55 GMT

I currently have 18 DHCP scopes setup on a Windows DHCP server. The only configuration required on the switches is on the Core switch that is doing the VLAN routing. That is the switch that has the interfaces on it that is the default gateway for clients. On each VLAN interface, you have to configure an IP Helper Address that is the IP address of the DHCP server. Once this is setup your VLANs will get directed to the DHCP server and get a correct IP address for the scope associated with that VLAN.

Syntax: ip helper-address num ip-addr

The num variable specifies the helper address number and can be from 1 through 16.

The ip-addr variable specifies the server IP address or the subnet directed broadcast address of the IP subnet the server is in.

Re: How to use Server based DHCP with ICX switches

r_gent — Fri, 12 Feb 2021 15:53:52 GMT

Thanks for the reply.

in terms of this, we have around 20 odd scopes specified on the windows dhcp. how would i get around selecting a subnet to be used?

say if vlan 120 is for one area and has the scope 192.168.120.0 how would i instruct this to be sent to the connecting switch?

and the address of the controlling server is 192.168.14.11

Re: How to use Server based DHCP with ICX switches

paul_mcguire_20 — Fri, 12 Feb 2021 16:00:35 GMT

config t

#interface ve 120
#ip helper-address 192.168.14.11

do this for each vlan virtual interface

Re: How to use Server based DHCP with ICX switches

tim_brumbaugh — Fri, 12 Feb 2021 20:59:31 GMT

So the thing to learn hers is that when a DHCP broadcast hits a switch with routing turned on and has an IP-Helper configured on the Virtual interface or VE it will forward that broadcast to the DHCP server. If that DHCP server has a pool of addresses that match the subnet from which the broadcast was forwarded from, it will hand one out to the device that is requesting the ip address even though the DHCP server lives on a different subnet.