DHCP Relay agent on Vlan

Posted on 2010-11-14
Last Modified: 2012-05-10

I have a topology as follows:

Router (Relevant sub interfaces for the vlan's  AND dhcp server for each vlan's network)  


Switch(With Multiple VLAN's AND 2 trunks: 1 for AP and 1 for the router))


Access Point (Cisco Aironet, broadcasting multiple SSID's, each assigned to a specific vlan).

Now if I add another vlan (example vlan 66) and want to host the dhcp server for that vlan somewhere else.

How can I tell the AP to send the dhcp requests coming on ssid of vlan 66 and switch ports of vlan 66 to a specific IP and not the router?

Will I still have a sub interface for vlan 66 on the router so clients on vlan 66 can talk to clients on other vlans?



Question by:masdf123
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LVL 79

Accepted Solution

lrmoore earned 334 total points
ID: 34133710
Yes, you will need a L3 subinterface on the router.
As long as the other DHCP server is connected to a VLAN66 access port and you do not setup DHCP on the router, the clients should get IP addresses no problem. Nothing else to configure.

Author Comment

ID: 34133716
so how does it work exactly?

When it doesn't find a dhcp server for that network (the sub interfaces network) it would automatically go to the other dhcp server?

How do I enable L3 on a sub interface?

Assisted Solution

joelvp earned 166 total points
ID: 34133926
You create a vlan interface on the router and use the ip helper-address command

So let's assume you're DHCP server has ip address and the subnet for vlan 66 is

You will create a DHCP scope for subnet on the DHCP server (

Then on the router, you can instruct the router to relay DHCP messages to this DHCP server for this specific vlan

interface vlan66
ip address 192.168.66.x
ip helper-address

By the way: by creating a vlan interface (as above) you in essence enable L3 on a subinterface (in switching terminology a subinterface would be called a vlan)

I think this is what you need?

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LVL 79

Assisted Solution

lrmoore earned 334 total points
ID: 34135698
If the DHCP server is in the same subnet as VLAN66, and connected to a switch port in static access vlan 66, there is nothing else to do because a VLAN is a broadcast domain.

If the DHCP server is on a different network, and has a scope for the 66 vlan subnet, then you use ip helper-address on the Vlan66 subinterface to act as a relay.

The L3 interface is the sub-interface with an IP address. This sub-interface IP would be configured as the default-router in the DHCP scope.

Author Comment

ID: 34135770
Thanks guys this clarify my question.

Author Comment

ID: 34136246
Another question I had was if a person gives himself a static and connect to the vlan 66 switchport...he would have access to the network. Basically vlan 66 has a cilli hotspot with free radius backwnd which also acts as a dhcp server. So after user authenticates chilli spot would replace the client gateway to the ip of vlan 66 sub interface.

So how can I prevent users from statically assigning an ip and connect straight to vlan66.

Expert Comment

ID: 34141539
you would need a cisco switch and use a feature called dhcp snooping in combination with ip source guard (ip verify source), but this seems a new question to me, doesn't it?

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