DHCP relay, superscope and laptops

Discussion in 'Server Networking' started by Pall Bjornsson, Apr 27, 2005.

  1. 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?

    Pall Bjornsson, Apr 27, 2005
    1. Advertisements

  2. 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.
    Phillip Windell, Apr 27, 2005
    1. Advertisements

  3. Pall Bjornsson

    Herb Martin Guest

    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".
    Herb Martin, Apr 27, 2005
  4. 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 ?

    Pall Bjornsson, May 4, 2005
  5. Pall Bjornsson

    Herb Martin Guest

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

    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?
    Herb Martin, May 4, 2005
  6. VLANs

    Same here, we use an HP 5300XL
    No. No Superscopes. VLANs don't change anything, L3 Switches don't change
    Phillip Windell, May 5, 2005
  7. Pall Bjornsson

    Herb Martin Guest

    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.
    Herb Martin, May 5, 2005
  8. 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 ?

    Pall Bjornsson, May 10, 2005
  9. Yep,...that's it
    Not the case. Absolutely not. No Superscopes!
    Phillip Windell, May 10, 2005
  10. Pall Bjornsson

    Herb Martin Guest

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

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

    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.
    Herb Martin, May 10, 2005
  11. Pall Bjornsson

    Todd J Heron Guest

    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.

    Todd J Heron, May 11, 2005
    1. Advertisements

Ask a Question

Want to reply to this thread or ask your own question?

You'll need to choose a username for the site, which only take a couple of moments (here). After that, you can post your question and our members will help you out.