C HAP TE R 10-1 Converged Plantwide Ethernet (CPwE) Design and Implementation Guide OL-21226-01, ENET-TD001E-EN-P 10 DHCP Persistence in the Cell/Area Zone Introduction This chapter describes the implementation of Dynamic Host Configuration Protocol (DHCP) persistence on an Industrial Automation and Control System (IACS) network and extends the design recommendations described in Chapter 3, “CPwE Solution Design—Cell/Area Zone,” Chapter 4, “CPwE Solution Design—Manufacturing and Demilitarized Zones and Chapter 5, Implementing and Configuring the Cell/Area Zone.” Table 4-7 highlights several ways to allocate IP addresses and lists advantages and disadvantages of these methods. Cisco and Rockwell Automation recommend that IACS network developers use a static IP addressing schema for the Manufacturing zone, especially for allocating IP addresses to IACS devices in the Cell/Area zone. Cisco and Rockwell Automation now recommend DHCP Persistence as a valid option along with static addressing for deploying IP addresses for IACS devices. As noted in earlier chapters, the Cell/Area zone is where the IACS devices connect into the Cell/Area IACS network. Careful planning is required to achieve the optimal design and performance from both the Cell/Area IACS network and IACS device perspective. This extension of the CPwE architectures focuses on EtherNet/IP, which is driven by the ODVA Common Industrial Protocol (CIP) (seethe “IACS Communication Protocols section on page 1-26 ). The EtherNet/IP protocol is tested with Rockwell Automation devices, IE switches, controllers, and applications. Static IP addressing is the traditional, default means to allocate IP addresses for both IACS devices for example, drives and IO) and network infrastructure devices (for example, IE switches. Static IP addressing requires an implementer to manually configure an IP address on an IACS device as it is provisioned onto the IACS network. Static IP addressing is referenced directly (rather than a logical reference) by the IACS applications for communication and control purposes. Therefore, the IP addressing assigned must be consistent and defined for proper IACS application operation. As IACS networks grow in size, so does the task of maintaining static IP addresses on IACS devices. During maintenance operations, where downtime cost and meantime to recovery (MTTR) is a significant issue, manual configuration of a static IP address for each replaced IACS device can take valuable time. DHCP Persistence enables IACS implementers to reserve and pre-assign an IP address to a specific IE switch port. This enables an IACS device connected to that IE switch port, configured for dynamic IP allocation, to always receive a consistent IP address regardless of its MAC address. This capability helps to reduce the amount of time required to provision or replace IACS devices, such as drives and IO. This also helps to reduce the required level of skilled resources to provision or replace an IACS device.
10-2 Converged Plantwide Ethernet (CPwE) Design and Implementation Guide OL-21226-01, ENET-TD001E-EN-P Chapter 10 DHCP Persistence in the Cell/Area Zone Introduction Although Cisco and Rockwell Automation now recommend DHCP Persistence as a valid option for IACS devices, Cisco and Rockwell Automation still recommend that network developers use a static IP addressing schema for IACS network infrastructure devices. This chapter outlines the key requirements and technical considerations for DHCP Persistence within the Cell/Area zone. There are two typical use cases for implementing DHCP Persistence replacement of a failed IACS device, and setting up anew “out-of-the-box” IACS device. Using DHCP Persistence to Replace a Failed IACS Device Consider the example of a municipal water distribution system that has multiple pumping stations located over a large geographic area. Often, these networks are tied together into a central location for monitoring purposes. Because of this centralization, it is convenient to have only a few network administrators who must maintain addressing for the entire system. If an IACS device on a pumping station fails, maintenance staff on site could replace the IACS device. However, special training in all IACS products maybe required to properly set IP addressing. If dynamic allocation is enabled on this IACS device, the maintenance staff would simply connect the new IACS device to the DHCP Persistence server (the IE switch to which the IACS device is connected, which allocates the correct IP address, enabling the maintenance staff to complete the IACS device configuration. Using DHCP Persistence to Provision a New IACS Device To reduce the amount of time necessary to configure anew system, Cisco and Rockwell Automation have enabled specific technology to allow a more efficient out-of-the-box experience when deploying IP-enabled devices in an IACS application. Manually configuring network addresses on IACS devices can add extra time and complexity to system setup. To configure DHCP, the following tasks must be performed: