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VLANs and NT based DHCP

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Nov 16th, 2001
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I am currently looking at the fesibility of running Microsoft NT4's version of DHCP over multiple VLANs. Could someone point me toward some viable documentation concerning this topic, thanks.. This will based on layer 2 VLANs....

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bsivasub Fri, 11/16/2001 - 06:24
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On the router interface, you need to configure

ip helper-address x.x.x.x


on all the vlans except where the vlan of the server.


you should be set.


This helper-address would turn routers DHCP relay agent and any broadcast from that segment would be unicast to the server and server would respond back to the router who would inturn pass it on to the actual request workstation.



morriszhu Sun, 11/18/2001 - 16:43
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but the issue would be you might get arbitray IPs for VLANs without separating them to individual Vlan groups.


I am wondering if it could be solved or not important for you.


Morris

patoil Sun, 11/18/2001 - 21:02
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Morris,

Don´t worry about getting arbitraty IP addresses in each VLAN. That won´t happen.

The broadcast DHCP request will carry information for the interface (net) that it had to pass through to get to the server, so that the server will know how to answer that request directly to the right network (or vlan). It´s all solved into the DHCP messages information. All you have to do is instruct your router not to drop DHCP broadcasts.

j.chenevey Fri, 11/16/2001 - 13:51
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This is very common. As noted in the previous reply, on the router (Cat5K RSM, Cat6K MSFC/MSM, or external) you need to configure "ip helper" statements directing the dhcp request to the server if the server resides in a different VLAN/subnet. The ip helpers forward a number of udp packets so you also have to explicitly configure it to forward only dhcp udp packets and not others (tftp, dns, time service, netbios, tacacs come to mind).


The commands to prevent flooding of those udp types by the ip helper are:


no ip forward-protocol udp [port number]

j-kurkjian Mon, 11/19/2001 - 08:42
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Here's a solution that worked great for me. Cat 6509 w/ SupII & MSFCII. Set up 4 IP/IPX VLAN's. Bought Intel Pro 100+ Server NIC's. The Intel NIC supports 802.1q VLAN's. Set up a NT 4.0 DHCP server and all scopes.Enabled 802.1q on the NIC(in the NIC Properties tab). Set the NIC up with the multiple VLAN's that I configured on the 6509. Set the port on the 6509 as a dot1q TRUNK and let it go. Give it a try, It works great and you have no need to foward DHCP on the router (or MSFC), the DHCP server is LOCAL to the VLAN.

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