DHCP relay, superscope and laptops

Hi !

I have a DHCP server serving a few seperate subnets. One subnet, which is
out of the superscope, is the one that the DHCP server is in, all the other
subnets are within the superscope. The other subnets connect through a
Level 3 Switch/Router which acts as a DHCP relay agent.

For most of the time everything works fine, until I have to take my laptop
and connect it to different subnets. My laptop has a fixed IP address,
assigned through DHCP as an indefininite reservation of IP address, on the
"primary" subnet, as there are some firewall filters to handle traffic to it
from both the internet, and the other subnets (for management purposes).

When I connect my laptop to any of the other subnets, which are in the
superscope, I get an IP address.

When I then connect it to the next subnet in the superscope, I don't get an
address via DHCP until I delete the assignment from the first subnet.

What I most would like to do is to manually reserve indefinitely one IP
address for my laptop in each subnet, which would be allocated via DHCP when
I connect to that subnet. That way I could set up some filter exceptions
that would only work for that IP address.

Thus, there are two questions:

1. How can I get this fixed IP address in each subnet to be allocated to my
laptop ?

2. How can I get an IP address allocated each time I connect to a subnet
without having to manually delete the first reservation?

Regards,
Palli

 

Get rid of the Superscope.

Get rid of the Superscope.

Get rid of the Superscope.

Get rid of the Superscope.

A separate Scope for each subnet. Get rid of the Superscope.

A separate Scope for each subnet. Get rid of the Superscope.

 

What's the superscope for?

Superscopes are only needed when the DHCP server offers
addresses to a MULTINET.

Multinet = more than one logical IP subnet on the same cable
segment, or more accurately, the "same broadcast domain."

Subnets on differnt cables should be in different scopes and
should NOT have a superscope.

Superscopes are groups of scopes on the same segment,
which is very unusual (not unheard of, but not very common.)

Start with deleting the superscope.

Before leaving a location you may (optionally) type:

ipconfic /release

Then when you move to a new location you can just type:

ipconfig /renew

(The renew will work without the release, but release is
a good idea if you pool of addresses on the first net is
relatively small -- universities have hundreds of people
leaving one subnet for another after every class, or when
leaving the dorms in the morning, etc.)

Modern machines (Win2000+) will usually figure it out if
you REBOOT, but the problem is more irritating with sleep
(hibernate or suspend).

You may have to perform the "ipconfig /renew".

 

Superscopes are only needed when the DHCP server offers

In my case I have a L3 Switch that splits my network in a few distinct VLANs
each of which has it's own subnet. The Switch then acts as a DHCP relay,
relaying DHCP requests to a central DHCP server, which then must offer IP
addresses for different subnets.

I was under the impression that if you had such a setup, you MUST have
superscope(s) set up, right ?

What we did as a workaround, was that we created one superscope for each
VLAN/Subnet and that seems to do the trick, but maybe that's not the way to
go ?

Regards,
Palli

 

Then each of these should be indistiguishable from
a hardward segment -- each VLAN forms a separate broadcast
domain.

If you only use one IP Subnet range on each VLAN,
you don't have multi-nets and thus don't need Superscopes.

It is precisely equivalent to ordinary subnets from
that point of view.

No, because you have no multinets.

Each DHCP server has a NIC on one subnet, and
the other subnets are are in different broadcast domains.

A Superscope wasn't necessary -- what problem were
you trying to solve?

 

VLANs

Same here, we use an HP 5300XL

No. No Superscopes. VLANs don't change anything, L3 Switches don't change
anything.

 

What Philip said -- just think of the routing (L3) between
different VLANS as passing through a router.

If effectively creates separate segments/cables -- the advantage it
you the admin get to define these logical segments/cables.

Once done, it is just like a segment with subnet on each one.


You would only need a superscope if you had multiple subnets
on a single segment/VLAN (that is called a "multinet".) You
need a superscope if the DHCP server will hand out addresses
to multiple subnets on such a Multinet.

 

OK, so to simplify...

I have two subnets A and B, one on each VLAN, connected through a L3 switch
which acts as a DHCP relay to my single DHCP server on subnet A.

A DHCP request from clients on subnet A are no problem as they are local on
the DHCP's server subnet.

A DHCP request from clients on subnet B, pass through the L3 Switch, which
relays them to my DHCP server on subnet A.

How do I then set up those two subents on the DHCP server? Do I just have to
seperate scopes and everyone is happy ever after ?

In the article I read then I needed to declare a superscope for subnet B for
the DHCP server to be able to handle it correctly. Is that not the case ?

Palli

 

Yep,...that's it

Not the case. Absolutely not. No Superscopes!

 

Yes. In DHCP a scope is (effectively) a subnet.

Technically, a scope supplies some portion of the addresses
to a single subnet.

No.

A superscope would only be necessary if there were two logical
subnes on the single physical VLAN-B (etc.) -- each subnet would
require a scope, but since they are on the same VLAN (same broadcast
domain) they would need to be combined into a superscope.

Your situation is PRECISELY equivalent to the admin with a router
separating two subnets (in regards to DHCP, relaying, and scopes.)

Your VLAN switch just sets up the two broadcast domains electrically
instead of directly physically.

 

In addition to Phil and Herb's comments, ensure that you configure an IP
helper address on the L3 switch (did you say it was a Cisco switch?) so
workstations on the other side can broadcast to the DHCP server. Or,
configure a DHCP relay agent somewhere in the subnet itself.

http://support.microsoft.com/kb/q190930/