Address Assignment through DHCP/BOOTP
Original instructions
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Introduction
This topic discusses the manner in which a BMX NOE 01x0 modules or the Ethernet port of a BMX P34 20x0 CPUs get an IP address from a DHCP/BOOTP server.
NOTE: Before attempting to get an IP address from a DHCP/BOOTP server, obtain the appropriate network address and subnetwork mask from your system administrator.
NOTE: The BMX NOE 01x0 module will not get an IP address from a DHCP/BOOTP server while downloading an application if the IP configuration has not changed.
Address Request and Reply
This table explains how a DHCP/BOOTP client (like an Ethernet module or CPUs with Ethernet ports) gets an IP address:
Step
Action
1
The module supports communications in the Ethernet II and 802.3 frame formats. (Ethernet II is the default.) When communicating with a DHCP or BOOTP server, the module makes 4 requests based on the configured frame type. To avoid flooding the server with simultaneous requests, the transmission time of each request is based on the backoff algorithms defined by the respective DHCP and BOOTP RFCs.
Note: In addition, modules that communicate with DHCP servers make requests in this order:
  • first: 4 requests in the DHCP format defined by RFC 2132
  • second: 4 requests in the DHCP format defined by RFC 1533
2
The remote device acting as the DHCP/BOOTP address server responds to this request and assigns to the client module:
  • an IP address
  • the gateway IP address
  • the corresponding subnetwork mask
Note: The display of this information from a browser connected on the HTML pages of the rack display on the web server is not constantly updated. Therefore, connect a Control Expert application on the correct PLC to obtain the actual values.
In the absence of an address server or if there is no server response, the client module can not get a stable IP address. Therefore, its services are not available. The client continues to issue periodic BOOTP/DHCP requests until it receives a response from the server.
NOTE: Setting the rotary switch position to Clear IP is one way to force the client module to use its default IP address to start basic services, including Modbus TCP messaging.
These concepts are graphically represented in the IP address assignment process flowchart.