Operation Diagnosis
236
9.9
Detecting IP Address Conflicts
UM Basic Configuration L3P
Release
7.1
12/2011
9.9 Detecting IP Address
Conflicts
9.9.1 Description of IP Address Conflicts
By definition, each IP address may only be assigned once within a
subnetwork. Should two or more devices erroneously share the same IP
address within one subnetwork, this will inevitably lead to communication
disruptions with devices that have this IP address. In his Internet draft, Stuart
Cheshire describes a mechanism that industrial Ethernet devices can use to
detect and eliminate address conflicts (Address Conflict Detection, ACD).
Mode Meaning
enable Enables active and passive detection.
disable Disables the function
activeDetectionOnly Enables active detection only. After connecting to a network or after an
IP address has been configured, the device immediately checks whether
its IP address already exists within the network.
If the IP address already exists, the device will return to the previous
configuration, if possible, and make another attempt after 15 seconds.
The device therefore avoids to participate in the network traffic with a
duplicate IP address.
passiveOnly Enables passive detection only. The device listens passively on the
network to determine whether its IP address already exists. If it detects a
duplicate IP address, it will initially defend its address by employing the
ACD mechanism and sending out gratuitous ARPs. If the remote device
does not disconnect from the network, the management interface of the
local device will then disconnect from the network. Every 15 seconds, it
will poll the network to determine if there is still an address conflict. If there
isn't, it will connect back to the network.
Table 30: Possible address conflict operation modes
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