connecting a device that supports IP address, but not subnet mask or gateway to a network with multi

Discussion in 'Cisco' started by Ned, Mar 29, 2007.

  1. Ned

    Ned Guest

    There are devices (time clocks) connected to our network that can be
    configured with an IP address but DO NOT support subnet mask or
    default gateway. Somehow they work, even though we are using multiple
    vlans and default gateways. Our current network consists of switches
    made by Marconi, I think they are ATM doing something called LANE (Lan
    emulation). I am moving to Cisco switches this weekend, 4507 at the
    core. Does anyone know if these devices will continue to work on a
    Cisco network without an option to configure a subnet mask and default
    gateway?
     
    Ned, Mar 29, 2007
    #1
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  2. Ned

    briggs Guest

    The key feature that you need to support this kind of device is
    proxy ARP. It is enabled by default (at least on the Cisco router I just
    tested on) under interface configuration mode.

    If you want to be doubly sure that it is enabled, you could go into
    interface configuration mode and turn it on, for instance:

    interface vlan37
    description An interface with brain-dead printers that need proxy ARP
    ip address 192.168.37.1 255.255.255.0
    ip proxy-arp


    interface vlan38
    description An interface with network gear made in the last ten years
    ip address 192.168.38.1 255.255.255.0
    no ip proxy-arp


    What these devices that have no subnet mask, a badly configured subnet
    mask or no default gateway will generally do is to assume that all
    target IP addresses are local. So they will send an ARP request.
    If there is a Cisco router on the segment and it has proxy-arp enabled,
    and if it has a route in its routing table toward the requested IP
    address, the router will respond to this ARP request with the MAC
    address of the router. The device will then forward the IP datagram
    to the Cisco router and the router will take care of delivery from
    that point on.
     
    briggs, Mar 29, 2007
    #2
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  3. Ned

    Ned Guest

    Thanks for the explanation.
    I'll test this out and post my results.
     
    Ned, Mar 29, 2007
    #3

  4. They probably have a subnet mask, but it probably assumes the subnet
    mask based on the ancient classful IP subnet definitions.

    They don't need to have a default gateway unless the packets need to
    travel over a layer-3 gateway, and you don't say enough about how your
    network is layed out to say if there are layer-3 gateways between
    segments.
     
    Doug McIntyre, Mar 30, 2007
    #4
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