Rockwell Automation Publication 1783-UM007G-EN-P - February 2017 241
Configure Switch Features Chapter 7
Reserve IP Addresses through DHCP Persistence
You can reserve and preassign an IP address from the IP address pool to a
specific switch port. As a result, a device that is connected to that switch port
always receives the same IP address regardless of its MAC address.
DHCP persistence is useful in networks that you configure in advance, where
dependencies on the exact IP addresses of some devices exist. Use DHCP
persistence when the attached device has a specific role to play and when other
devices know its IP address. If the device is replaced, the replacement device is
assigned the same IP address, and the other devices in the network require no
reconfiguration.
When the DHCP persistence feature is enabled, the switch acts as a DHCP
server for other devices on the same subnet, including devices that are
connected to other switches. If the switch receives a DHCP request, it
responds with any unassigned IP addresses in its pool. To prevent the switch
from responding when it receives a request, check the Reserve Only box on the
DHCP window.
When DHCP persistence is enabled and a DHCP request is made from a
connected device on that port, the switch assigns the IP address for that port. It
also broadcasts the DHCP request to the remainder of the network. If another
DHCP server with available addresses is on the network and receives this
request, it can try to respond. The response can override the initial IP address
the switch assigns depending on how the end device behaves (takes first IP
address response or the last). To prevent the IP address from being overridden,
enable DHCP snooping on the appropriate VLAN. Enabling DHCP
snooping blocks the broadcast of this DHCP request, so that no other server,
including another Stratix switch with DHCP persistence enabled, responds.
If you are using DHCP persistence, we recommend that you initially assign
static IP addresses to end devices. If an end device fails and is replaced, the
DHCP persistence feature assigns an IP address from the DHCP persistence
table. The device functions properly with this IP address, but we recommend
that you reassign a static IP address to the replaced devices.
IMPORTANT DHCP persistence does not work with DHCP for ring devices, as described on
page 222. Configure only one of these features on your switch.
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