VLANS and DHCP

[Joseph Huang 黃立行]

If your routers are cisco like
just add "ip helper-address dhcp.server.ip.address" configuration in
each vlan interface
like:
Switch>enable
Switch#configure terminal
Switch(config)#interface vlan 1968
Switch(config-if)#ip helper-address 192.168.100.200
Switch(config-if)#end

then the router will relay dhcp packets between dhcp server and dhcp client

if you want to use a dhcp server with many virtual interface to do this
you need
1. a nic with 802.1q vlan support on your dhcp server
2. define virtual interfaces on your dhcp server machine
3. define a trunk port which directly connect to dhcp server's nic and
brings all vlan tag to the nic of dhcp server
cisco like config is :
Switch>enable
Switch#configure terminal
Switch(config)#vtp mode transparent
Switch(config)#interface gigabitEthernet 0/12
Switch(config-if)#switchport trunk encapsulation dot1q
Switch(config-if)#switchport mode trunk
Switch(config-if)#end

4. tell dhcp server to listen dhcp packets on virtual interfaces

however, maybe nic won't support more than 64 vlans
so, i think you need both router's dhcp relay and dhcp server's virtual
interface configuration
(mix L2 and L3 traffic on the router or core switch)

finally, you can download cisco's "Multilayer Switch Software
Configuration Guide" of their products
on cisco's web site, their manual is really easy to understand.

good luck!

Joseph Huang


Steve Wenger wrote:
> I will be running it on Fedora Core 5,  I am going to have approx 100 total 
> vlans to start, and I know it can be done I am just not sure how to get it 
> done.
>
> Steve Wenger
> Nconnect Internet and High Speed Wireless.
> Got a need for speed?  We have a solution for you!
>
>
> -----Original Message-----
> From: Michael Loftis <mloftis at wgops.com>
> To: dhcp-users at isc.org
> Date: Thu, 17 Aug 2006 20:27:59 -0600
> Subject: Re: VLANS and DHCP
>
>   
>> --On August 17, 2006 9:09:53 PM -0500 Steve Wenger 
>> <steve at nconnectwireless.com> wrote:
>>
>>     
>>> I am currently running a wireless network with over 2ooo clients
>>>       
>> attached
>>     
>>> to  50 different access points,  I will be reconfiguring the network
>>>       
>> and
>>     
>>> setting  up different VLANS for each tower location.  I want to use
>>>       
>> one
>>     
>>> centralized  DHCP server so I need to configure a DHCP server that
>>>       
>> can
>>     
>>> assign clients  addresses from multiple address blocks both public
>>>       
>> and
>>     
>>> private based upon  the VLAN that the client is part of.  Can anyone
>>> point me to some  documentation on how to accomplish this.
>>> Steve Wenger
>>> Nconnect Internet and High Speed Wireless.
>>> Got a need for speed?  We have a solution for you!
>>>       
>> What platform?  Linux and FreeBSD both support an 802.1q/VLAN
>> encapsulation 
>> driver.  I've used both to run DHCP and they work very well.  They both
>> expose an interface to DHCPd, so there's nothing 'special' as far as
>> DHCPd 
>> is concerned.  In Linux there's an option you have to sometimes set
>> that 
>> makes sure the Layer2 Ethernet headers are correct in order to make
>> certain 
>> the DHCPd works.  In any case it's not the DHCPd that does the VLAN 
>> interface, it's the host OS.
>>
>>     
>
>
>
>
>