Re: connecting a device that supports IP address, but not subnet mask or gateway to a network with multiple vlans
- From: briggs@xxxxxxxxxxxxxxxxx
- Date: 29 Mar 2007 14:47:14 -0500
In article <1175195796.721656.266050@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>, "Ned" <nedhart@xxxxxxxxxxx> writes:
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?
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.
.
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