Inter-Vlan Routing & DHCP - Cannot obtain an address

Discussion in 'Cisco' started by Damo, Feb 19, 2004.

  1. Damo

    Damo Guest

    I cannot obatin a DHCP address from the server when on a VLAN
    different to the DHCP server.

    I have a Cisco network with several buildings joined via fibre.

    The core building has a single 3750 12 Port fibre switch that
    terminates each of the fibre connections from the other buildings. It
    also connects directly to the sever. Each building connection is
    configured as a trunk with dot1q encapsulation

    The outer buildings all have 3550-24/48 switches with a Gigabit SX
    FIbre GBIC.

    I have configured VLANing and each building has its own VLAN. I have
    given an IP address to each VLAN on the 3750 and this is the default
    gateway for each of the respective clients. I have also configured "ip
    helper-address" on each VLAN interface and pointing it at the single
    DHCP server.

    The DHCP server has seperate scopes configured for each of the VLANs.
    If I manually assign an IP address to a client machine in a seperate
    VLAN to the DHCP server, I can ping each of the other vlans and all
    the servers with no problem.

    As soon as I set the client to DHCP I cannot obatin an address despite
    the ip helper-address command being used on that VLAN and the fact I
    can ping the DHCP server when I have a static address.

    Any ideas?? Any help would be appreciated.
    Damo, Feb 19, 2004
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  2. Damo

    chris Guest

    Anything in the logs of the DHCP server?

    Have you run a packet sniffer to see what's happening? Run it at the
    DHCP server and on the client. A great free sniffer can be found at It's available for windows, linux, etc and can
    really help you figure out what's happening. For example is the
    request not getting forwarded, is the reply a nack, is the reply not
    getting back to the client, etc.

    chris, Feb 20, 2004
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  3. Damo

    Paul Hardy Guest

    are the ports set to portfast, I have had trouble with machines trrying to
    get a lease before the network is up
    Paul Hardy, Feb 20, 2004
  4. Damo

    Damo Guest

    Thanks for the responses.....

    All the client ports are set to portfast.

    I have yet to try a sniffer on the network however thanks for the
    link, I will give this a try.

    I have found several Cisco documents on this topic however the network
    I am configuring this on is setup the same as in the documents.

    I will try the sniffer during the week and post any results I get from
    Damo, Feb 20, 2004
  5. Damo

    Damo Guest

    OK, I have run a sniffer on the network with the client plugged into
    the default VLAN (Obtains DHCP address instantly) and with the client
    plugged into a port on a seperate VLAN.

    The results:

    For the example:
    Client MAC =
    DHCP Server MAC = - IP =

    Default VLAN (Obtains DHCP) -> ff.ff.ff.ff.ff.ff ARP (Who has -> ARP (Address is -> DHCP (DHCP REQ) -> DHCP (DHCP ACK)

    Seperate VLAN (Does not obtain DHCP) -> ff.ff.ff.ff.ff.ff ARP (Who has

    This is indicating to me that the ip helper-address is working as the
    client is broadcasting for the MAC of the correct IP address of the
    DHCP server however it is not getting a response. The default gateway
    of the DHCP server is the backbone switch and the DHCP server can ping
    the VLAN IP addresses. Once again if I give the client a static
    address in th ecorresponding VLAN IP range it works fine.

    There is nothing in the logs in relation to the DHCP server.

    Any ideas??
    Damo, Feb 24, 2004
  6. Damo

    Hansang Bae Guest

    Why is your PC arp'ing for someone outside of his subnet? I'm assuming
    the server is at but your PC is not (otherwise you wouldn't
    need a router).



    "Somehow I imagined this experience would be more rewarding" Calvin
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    Hansang Bae, Feb 25, 2004
  7. The client thinks it already knows its ip address and it's just
    confirming it with the server, whose address it also already knows.

    The client is renewing an existing lease rather than requesting a new
    The client thinks it already knows its ip address and it's just
    confirming it with the server, whose address it also already knows. This
    time it doesn't work because there is no answer to the arp, because we are
    no longer on the same vlan.

    What the client should do here is give up trying to contact the server
    directly, and go back to the beginning and broadcast a DHCP DISCOVER,
    which will be forwarded by the ip helper on the router and the server will

    Windows, some versions, is known for misbehaving in this respect. You
    have to do ipconfig /release, ipconfig /renew if you move vlans.
    This has nothing to do with ip helper address. Ip helper address
    forwards udp broadcasts, which arp is not. Arp is local to the vlan and
    doesn't cross routers.
    Tell your client to forget wahtever it thinks it knows about its existing
    ip address and start the process from the beginning by broadcasting a DHCP
    DISCOVER, and you stand a better chance of success.
    Martin Gallagher, Feb 25, 2004
  8. Damo

    Damo Guest

    Thanks for your detailed responses. I revisted this yesterday to
    obtain a packet trace from the DHCP server rather than from the

    I started the sniffer and attempted to obtain an adress to capture the
    failure but this time it didnt fail !! I obtained an address from the
    DHCP server in the correct VLAN scope. AND NOTHING HAS CHANGED !!! :s

    The only difference is I tried what Martin suggested and I did a full
    release before the renewal. So to prove this theory I connected a
    second machine to this VLAN and did not do a release just a straight
    renew and it also worked!

    I then thought it had something to do with plugging teh DHCP server
    into a hub to do the packet sniffing so I plugged it back into the
    switch, added a third machine to the VLAN did a renewal and it also

    I am glad that this problem has rectified itself but at a loss to why
    it hasnt worked for two weeks and all of a sudden with no changes it
    works. I am still investigating it as I have a number of additional
    VLAN's to setup and would like to know the cause of the original
    problem in case it reoccurs on the other VLAN's.

    As suggested by Martin after the DCHP server fails to respond to the
    confirmation it should go back to the beginning and broadcast a DHCP
    DISCOVER. This was not occurring until after I did a release on one

    One possible suggetsion is that these machines all have a Novell
    client on them which may be interfering with the process.....only
    possible explanation I can come up with at this time as within the
    client there is a configuration option for DHCP so it may be trapping
    the request and redirecting it.
    Damo, Feb 27, 2004
    brad.hokanson, Dec 16, 2004
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