Troubleshooting ARP

Network traffic sometimes fails because a router's proxy ARP request returns the wrong address. A router makes this ARP request on behalf of an IP address on its intenal subnets (just as a remote access server makes a request on the LAN for its remote access clients). The problem is that the router's proxy ARP requests return the wrong MAC address to the sending host. As a result, the sending host sends its traffic to the wrong MAC address. In other words, the problem stems from proxy ARP replies.

To address this problem, use Network Monitor to capture a trace. If the trace reveals that when a sending host sends an ARP request for the MAC address of a destination IP address, a device (usually a router) replies with a MAC address other than the destination's correct MAC address.

To determine if this is the problem, check the ARP cache of the source host to make sure it is getting the correct IP address to MAC address resolution. Alternatively, you can capture all traffic with Network Monitor and later filter the captured traffic to display only the ARP and RARP protocols. The RARP protocol converts MAC addresses to IP addresses and is defined in RFC 903.

You can fix the ARP problem by disabling 'Proxy ARP' on the offending device. Exactly how this is done depends on the device's make and model; consult the manufacturer's documentation.

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