Using DHCP and vlans

Unanswered Question
Feb 15th, 2008

Currently we have a dhcp server that

issues 192.168.1.x addresses to everyone on our network-we only have 1 vlan. We are trying to test a new data link that would Utilize a .21 gateway.

So I would have to create vlan 21 membership on the switch-and assign ports to whom i want to be on the .21.x network. Is it possible to have a DHCP server giving out mutliple networks (.1.x and .21.x)

I.E...everyone who is on the 21 vlan

would get a dynamic address of 192.161.21.x with a .21.x gateway.

and everyone who is on the 1 vlan would still get the address

all residing on the same physical switch


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royalblues Fri, 02/15/2008 - 11:32

Yes it is possible to have a dhcp server lease out mutiple networks

In these cases, the dhcp relay is configured on the SVI or the gateway which sends the unicast request to the DHCP server.

Do you have any exisitng layer 3 switch/router apart from the mentioned 2950 switch?


lamav Fri, 02/15/2008 - 11:35


The answer is yes.

You can configure separate user VLAN interfaces for the different user subnets.

When an end-machine is plugged into a switch port, it will send out an L2 broadcast, known as a DHCP Request, on the VLAN for which that port is configured.

The assumption, of course, is that these DHCP servers are reachable from within that VLAN. If not, you would have to configure helper addresses for the different DHCP servers under the L3 VLAN interface configuration and have the router forward the DHCP Request packet for the users.



brooklynheight Fri, 02/15/2008 - 12:01 L3 switch...Just a series of 2950's connected to each other.

since i only have 2950's im assuming this cant be done?

royalblues Fri, 02/15/2008 - 12:08

If you have 2 NIC on your server, you can try placing each one of them in one vlan.

BTW, what is the use of the 2 scopes/vlans when there is no L3 device? THese 2 subnets cannot communicate between each other unless the packets are routed through a L3 device


royalblues Fri, 02/15/2008 - 12:55

configuring the helper address on the router interface would do the trick

service dhcp


ip add 192.168.21.x

ip helper-address



brooklynheight Fri, 02/15/2008 - 13:01 I said we have multiple 2950's all linked to one 2950 mdf..on the MDF 2950

we actually have 3 seperate routers connected to it. Our DHCP actually hands out 2 diff gateways to each host in the 192.168.1.x network.

Depending on the destination address, each

gateway will handle the desetination traffic.

Im trying to figure out where DHCP is configured.

should i be looking only for the ip helper addres in one router statement?


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