dhcp to match vlan

[pwallace]

We are implementing vlans within our network. We have vlan routing in the switches working and have now moved on to test dhcp for each vlan. the microsoft team has set up a superscope on the test dhcp server and when a workstation say from vlan 10 tries to get a dhcp address it passes out a 10.0.5.2 for example but it also passes out a 10.0.5.3 for vlan 20 which is a network say 10.0.8.0. I read somewhere not to use a superscope in doing this but the microsoft guys say we have to has anyone ran into this issue.

[Roberto Salazar]

I am not familiar with super scope, however, I can explain the vlan concept. Think of a vlan in subnet terms, for example, vlan 20 would be subnet 10.20.20.0/24 which gives you 254 hosts. If you host is this subnet, you will need to be routed to get to other subnet or vlan, which is the job for the L3 device or default gateway in hosts perspective. This subnet is also a broadcast domain, broadcast done by a host in this vlan/sunet would not be propagated to other subnet/vlan. If you're network is segregated by vlan and you are doing intervlan routing. I do not understand how and why it would makes sense to assign an IP address for vlan 20 when the host is in vlan 30 or any other vlan. Because from your description a host in subnet/vlan 10.0.8.0 (what is the subnet mask for this network?) would be assigned an ip address an ip address of 10.0.5.3 (not knowing the address mask, I would assume it's in the different network). How, is your vlan subnetted? The DHCP scope would have to defined according to your subnetting.

[pwallace]

We have the vlans subnetted as vlan 20 10.0.5.0/24

vlan 30 as 10.0.6.0/24. I can go from vlan to vlan with the intervlan routing that part is working fine. The microsoft guys have the networks on the dhcp server defined but the dhcp server still hands out ips from one the very first zone to each vlan the 10.0.5.0/24 network.

[andrew.butterworth]

DHCP SuperScopes are designed for networks where you have multiple IP networks/subnets on the same layer-2 network. The best example of this is when you use secondary ip addresses on a router interface, i.e.

interface FastEthernet0/0

ip address 10.0.5.254 255.255.255.0

ip address 10.0.8.254 255.255.255.0 secondary

ip address 10.0.10.254 255.255.255.0 secondary

A DHCP superscope containing networks 10.0.5.0/24, 10.0.8.0/24 and 10.0.10.0/24 should be created for the above configuration, But:

THEY WILL NOT WORK in your scenario.

I see this all the time with so-called 'clever' server guys configuring this and then the network guys spending ages looking in the wrong places trying to find the problem

Remove the SuperScope configuration on your DHCP Servers and use unique scopes per VLAN and it will work. By all means use global server options such as domain name, dns etc (NOT Gateways....), but SuperScopes in a routed environment DON'T WORK.

Some info here:

http://www.cramsession.com/articles/files/dhcp-server-superscopes-9172003-0848.asp

HTH

Andy

[Roberto Salazar]

Thanks, well put.

[pwallace]

Thank for pointing me in the right direction. I had read this some where befoe but did not bookmark it. wanted to be sure before I went back to the microsoft side on the way we should be headed. Thanks again have a great weekend