Allen-Bradley armorstratix 5700, User Manual, Page: 127 / 458

Rockwell Automation Publication 1783-UM007I-EN-P - December 2017 127
Configure Switch Features Chapter 3
Dynamic Host Configuration
Protocol (DHCP) Persistence Every device in an IP-based network must have a unique IP address. DHCP
assigns IP address information from a pool of available addresses to newly
connected devices (DHCP clients) in the network. If a device leaves and then
rejoins the network, the device receives the next available IP address, which is
not necessarily the same address that it had before.
The switch can be set to operate as a DHCP server to provide DHCP
persistence. With DHCP persistence, you can assign a specific IP address to
each port to make sure that a device that is attached to a specific port receives
the same IP address. This feature works with only one device that is connected
to each port configured for DHCP persistence. The DHCP server also serves
addresses to BOOTP clients.
You can assign an IP address from the IP address pool to a specific switch port.
A device that is connected to that switch port always receives the address that
you assigned to the port regardless of its MAC ID.
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 keep the switch from
responding when it receives a request, check the Reserve Only box on the
DHCP page.
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 keep 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.
IMPORTANT To make sure DHCP persistence works correctly, follow the application rules.
IMPORTANT DHCP persistence does not work with DHCP for ring devices as described on
page 111 . Configure only one of these features on your switch.