DHCP on 3825 for Specific VLAN

Scott Hanson · ‎08-05-2013

Hello all,

I am trying to configure a 3825 as a DHCP server but I only want it to hand out IP addresses to clients on a specific VLAN. Is that possible and if so can someone send me a link?

Thanks in advance! Replies rated.

cadet alain · ‎08-05-2013

Hi,

just create a scope for the specific subnet of that VLAN and also if other subnets(VLANs), don't forget to add a route for the IP address of the DHCP relay interface on that VLAN because the router will send a unicast offer to the Relay agent which then will unicast or broadcast the offer.

Regards

Alain

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John Blakley · ‎08-05-2013

Are the vlans on a l3 switch that you're routing for, or is this router configured as router on a stick?

If you have a l3 switch, you'd create your dhcp server on the router and then point the svi to the router to pull addresses from:

On the router:

ip dhcp excluded-address 192.168.10.1-192.168.10.99

ip dhcp pool ForVLAN10

network 192.168.10.0

default-router 192.168.10.5

dns-server 4.2.2.1

On L3 Switch:

int vlan 10

ip address 192.168.10.5 255.255.255.0

ip helper-address 192.168.1.1 <---router address

The request will match the 192.168.10.0/24 subnet.

If you have it configured on the router with subinterfaces for router-on-a-stick:

ip dhcp excluded-address 192.168.10.1-192.168.10.99

ip dhcp pool ForVLAN10

network 192.168.10.0

default-router 192.168.10.5

dns-server 4.2.2.1

int fa0/0

ip address 192.168.1.1 255.255.255.0

int fa0/0.10

encapsulation dot1q 10

ip addres 192.168.10.1 255.255.255.0

You won't have to do anything more other than add the dhcp pool at the top.....

HTH,
John

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HTH, John *** Please rate all useful posts ***

Scott Hanson · ‎08-05-2013

Hello,

the switches are Layer 2. Are you both saying I need to configured the DHCP on the router? I can do that no problem. How do I restrict it into only handing out IP addresses for that 1 particular VLAN?

cadet alain · ‎08-05-2013

Hi,

how is your dhcp server configured and what device is this ? what is your topology ?

Regards

Alain

Don't forget to rate helpful posts.

Scott Hanson · ‎08-05-2013

Hello,

per the problem description I would like to make a 3825 router the DHCP server. That router is plugged into a 2960 switch that is part of a stack. I have LWAPs hanging off trunk ports on the switches. My goal is to make 1 particular SSID an "Internet Only" SSID so that it gets an IP address from the router instead of the internal DHCP server. I also want to segment the traffic as "Internet Only" so that they don't touch the Internals network.

John Blakley · ‎08-05-2013

Scott,

Do you currently have subinterfaces configured on the router now to support your vlans?

HTH,
John

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HTH, John *** Please rate all useful posts ***

Scott Hanson · ‎08-05-2013

Hello JOhn,

I do curently have sub-interfaces. I configured a new one for this "Internet-Only" VLAN

John Blakley · ‎08-05-2013

Okay. All you need to do then is create a pool that corresponds to your subinterface that you want users to be able to get the address from like in my first post.

ip dhcp excluded-address 192.168.10.1-192.168.10.99

ip dhcp pool ForVLAN10

network 192.168.10.0

default-router 192.168.10.5

dns-server 4.2.2.1

int fa0/0

ip address 192.168.1.1 255.255.255.0

int fa0/0.10

encapsulation dot1q 10

ip addres 192.168.10.1 255.255.255.0

Your other users will land on an interface that probably has a helper configured on the interface for the other vlans minus the one that your dhcp server is actually on.

HTH,
John

*** Please rate all useful posts ***

HTH, John *** Please rate all useful posts ***