Catalyst 2950

Discussion in 'Cisco' started by Pondo, Dec 24, 2004.

  1. Pondo

    Pondo Guest

    Hi Everyone!
    Would you please help me out with this:
    How to set up a switch 2950 to assign dynamic ip to host to connect to
    the internet.
    I set up the router (2611) with ip dhcp pool, network, default-router
    and dns server but I cannot make the switch give dynamic ip to the
    hosts from the router pool to connect to the internet. Hosts get a
    169.254.x.x ip, but the dhcp pool is 172.16.x.x
    I thank you.
    Pondo, Dec 24, 2004
    #1
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  2. In article <>,
    Pondo <> wrote:
    :Would you please help me out with this:
    :How to set up a switch 2950 to assign dynamic ip to host to connect to
    :the internet.
    :I set up the router (2611) with ip dhcp pool, network, default-router
    :and dns server but I cannot make the switch give dynamic ip to the
    :hosts from the router pool to connect to the internet. Hosts get a
    :169.254.x.x ip, but the dhcp pool is 172.16.x.x

    You wouldn't use a switch for that purpose.

    DHCP works by having the requesting host send out an UDP
    bootp packet to the all-hosts IP address (255.255.255.255);
    as per usual, that IP address is implimented at Layer 2 by
    a destination MAC address which is all binary 1's
    (FF:FF:FF:FF:FF:FF). The DHCP server is expected to be in the
    same broadcast domain; packets addressed to 255.255.255.255
    are usually not forwarded across router interfaces [and in
    this case it wouldn't help if they were], but the packets
    are usually flooded by any switches to all ports on the same
    VLAN. The DHCP server hears the broadcast and replies directly
    to the originating system by extracting the source MAC address
    from the bootp request and using that as the destination.
    The response contains an IP allocation proposal; the originating
    machine picks one of the proposals it receives back and acknowledges
    that one to complete the transaction.

    Notice that in this sequence that the switch itself does not allocate
    IPs, does not act as any kind of active agent in the DHCP sequence and
    doesn't need to know anything at all about DHCP: it just has to pass on
    broadcast packets as per its switch rules.


    In some cases, the restriction that the DHCP server must be in the
    same broadcast domain can become impractical, as it implies that
    there must either be distinct DHCP servers for each VLAN, or that
    DHCP servers must have interfaces in each VLAN. This can become
    particularily problematic when a VLAN does not correspond to
    a unique IP subnet -- it is quite legal to have different VLANs
    involving the same subnet. You can't route between the VLANs if
    the IP ranges overlap, but you don't necessarily *want* the distinct
    VLANs to be able to communicate with each other.

    To assist in such cases, some switches allow you to configure
    a DHCP "helper address", which is the IP address of a DHCP server.
    When the switch detects a DHCP request packet, it forwards the packet
    to the configured IP address, which might be in a different
    broadcast domain; the switch might amend the request along the way
    to add in information about which device and port it detected the
    request on. This forwarding of the DHCP request is really a Layer 3
    operation, so a Layer 2 switch such as the 2950 won't necessarily
    impliment the DHCP helper feature.

    Even when the switch does allow a DHCP helper address, the switch does
    not mediate the DHCP sequence or act as a DHCP proxy. That is, the
    switch does not take an active part in allocating the IP address or in
    deciding which responding DHCP server's proposal is to be preferred.
    Thus, one does not "set up a switch 2950 to assign dynamic ip to host":
    the 2950 will NOT assign dynamic IPs to hosts. At most, the 2950 will
    pass along the message to servers that do the actual IP allocation.

    If your devices are not getting assigned an IP address and so are
    chosing a 169.254 address as a last resort, then it is because the
    DHCP server (the 2611) cannot hear the request for some reason
    (e.g., it is in a different VLAN) or else because the DHCP server
    is not properly configured to hand out IPs to anything.

    If you want the switch itself to hand out dynamic IPs then you
    need a layer 3 switch such as the 3550 or 3750; and even then keep
    in mind that if you have multiple DHCP servers configured to hand
    out the same IP address range, then the DHCP servers will NOT
    automatically coordinate IP allocations, so you will end up with
    multiple devices assigned the same IP. If you want coordinated
    IP allocation from multiple DHCP servers, then the servers have
    to be designed to do that; the DHCP servers built in to Cisco
    routers and multilayer switches do not have that functionality.
    --
    Entropy is the logarithm of probability -- Boltzmann
    Walter Roberson, Dec 24, 2004
    #2
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