 Dynamic Host Configuration Protocol (DHCP)
Dynamic IP Address Allocation
Dynamic Host Configuration Protocol (DHCP) is used widely in enterprise network applications to automatically assign Internet Protocol (IP) addresses to equipment connected to these networks. A computer, router or switch with the capability to act as a DHCP Server can issue IP addresses dynamically from a range or "pool" of addresses programmed into it. The server responds to a DHCP Request from a DHCP Client with an IP address which it "Leases" to the client for a lease time. Lease time is a parameter set in the DHCP Server that controls when the client must reapply for an IP Address. Some time before the DHCP Lease is scheduled to expire the DHCP Client must renew the lease with the server by sending another DHCP Request. When equipment is removed from a network its IP address will be made available to another network device after the existing lease time expires.
Option 82 Relay Agent IP Address Allocation
Dynamic IP allocation is useful any time that the network equipment does not require IP addresses to remain unchanged over time. In most industrial process control installations, however, the IP address of all of the control devices must remain the same. Control network components must maintain addresses that are registered with their controller. The controller is sending and receiving data to and from field devices and has been given the IP location (address) of each device it is controlling as part of its software setup. The IP relationship between a controller and an I/O rack connected in the network must remain constant. If an I/O rack is replaced the new rack must be assigned the same IP address as the one it is replacing or the controller will not be able to communicate with it.
The need to maintain the IP Address for field devices can be accomplished using DHCP Option 82 Relay. With DHCP Option 82 Relay the server works with other switches configured to be Relay Agents to assign IP Addresses to specific Relay Agent switch ports. If an I/O Rack is replaced on a specified Relay Agent switch port the replacement I/O Rack will receive the same IP address previously programmed in the DHCP Server for that port. In this manner control devices can be replaced by maintenance personnel without the need to program an IP address in the unit. As long as the replacement unit is configured as a DHCP Client and connected to the correct Option 82 Relay Agent switch port it will receive the proper IP Address. This function can simplify field maintenance and reduce the recovery time caused by failed field equipment.
A switch setup to be a Relay Agent may not receive its IP address using Option 82 if it is connected to the DHCP Server via another Relay Agent.
Option 61 IP Address Allocation
Option 61 allows the DHCP Server to be setup to provide a preprogrammed IP address to a field device based on its Media Access Control (MAC) Address or an ID entered into the field equipment. This option will allow the equipment to move from one part of the network to another while maintaining the same IP address. Option 61 is useful when test equipment is moved from one part of the plant to another but must maintain the same IP Address. It can also be used for IP telephone systems allowing the phone to be relocated while maintaining the same Phone number (ID).
Port or Local IP Address Allocation
This form of IP Allocation doesn't require a full featured DHCP Server to be present. The port or local IP allocation is assigned per port as part of the switch setup. The switch will intercept the DHCP request from the field device connected to the port and respond to the request with the IP address provided for that port.
DHCP IP Address Allocation in Ring Topology
N-Ring and Rapid Spanning Tree ring management protocols are fully compatible with all DHCP address allocation methods. When N-Ring Topology is used, only one N-Ring manager is allowed in the ring. N-Ring redundancy can be accomplished using N-Link to couple two N-Rings.
Redundant DHCP Server
Two DHCP servers can be configured identically with Option 82 and Option 61 Address allocation so that if one server is lost the other will be available to provide IP addresses to the field devices. Redundant DHCP Servers should not be configured to supply Dynamic or Pool Addressing as this will cause the two servers to issue conflicting IP Addresses. If dynamic range address allocation is needed another switch in the network can be setup to handle this function.
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