Figure 15: Cpe330 Overlapping Local Ip Subnet Example - Emerson PACSystems RX3i User Manual

PACSystems™ RX3i and RSTi-EP CPU Reference Manual
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SRTP, which is proprietary and which provides the interface with the PME
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programming and configuration software and supports communications with
certain control systems and supervisory computer layers in the factory;
Modbus/TCP, which supports the Modbus messaging structure over the Internet.
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On the CPE302/CPE305/CPE310 models, the same shared processor performs both
Ethernet port processing and Controller logic processing.
On the CPE330, the dual core CPU enables communication to be handled by one core
while CPU logic and I/O scanning is handled by the second core. Furthermore, each LAN
interface is controlled by a dedicated Network Interface Controller (NIC). In the CPE400
and CPL410, one of the four microprocessor cores is dedicated to handling the
communications function (LAN1, LAN2 and LAN3).
As a result of the hardware advances in the CPE330, CPE400 and CPL410, a higher level of
processing power is provided in support of each LAN. This is especially important at
higher data rates. It also offloads the handling of Ethernet-level activity from the
processor core tasked with performing CPU logic and I/O scanning, permitting that core
to run more efficiently.
Each interface on a LAN must have a unique IP Address and a non-overlapping IP subnet.
This is configured in PME. Care must be taken to survey the entire connected network
architecture in order to tabulate the IP addresses and IP subnets already in use, both on
the local networks and on any of its routed subnets connected with a gateway. Never
assign a conflicting IP Address or configure duplicate IP subnets.
The following examples would be problematic:
5.1.1.1.1 Problem example #1:

Figure 15: CPE330 Overlapping Local IP Subnet Example

The issue demonstrated in is that requests entering one CPE330 interface can be routed
out the other interface since both CPE330 Ethernet ports have been configured to be on
the same network (255.255.0.0) but are physically connected to separate networks.
Avoid this by assigning non-overlapping Subnets.
Communications
Section 5
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