Allen-Bradley armorstratix 5700, User Manual, Page: 128 / 458

128 Rockwell Automation Publication 1783-UM007I-EN-P - December 2017
Chapter 3 Configure Switch Features
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.
The following figure and table illustrate DHCP persistence behavior.
FA1
FA2
FA3
FA7
FA4 FA5 FA6 FA8
Switch 1
Switch 2
Table 46 - DHCP Persistence Behavior
If Then
• Switch 1 has ports FA1…FA3 in its persistence table
• Switch 2 has ports FA4, FA5, FA6, and FA8 in its persistence table
• Reserve Only is not selected and DHCP snooping is off
A new device that is connected to switch 1 FA1 receives an IP address from the switch 1 persistence table.
A broadcast request is also sent across the network. Switch 2 responds if there is an unassigned address in
its pool. The response can override the assignment that is made by switch 1.
• Switch 1 has ports FA1…FA3 in its persistence table
• Switch 2 has ports FA4, FA5, FA6, and FA8 in its persistence table
• Reserve Only is selected in both switches and DHCP snooping is off
A new device that is connected to switch 1 FA1 receives an IP address from the switch 1 persistence table.
A broadcast request is also sent across the network. Switch 2 does not respond to the request. If the
device is connected to FA7 of switch 1, it does not receive an IP address from the switch pool because it is
not defined in the persistence table. Also, unused addresses in the pool are blocked.
• Switch 1 has ports FA1…FA3 in its persistence table
• Switch 2 has ports FA4, FA5, FA6, and FA8 in its persistence table
• Reserve Only is selected in switch 1 and DHCP snooping is off, but
not switch 2 when DHCP snooping is off
A new device is connected to FA1 receives an IP address from the persistence table. A broadcast request is
also sent across the network. Switch 2 does not respond to the request. In addition, a device that is
connected to FA4 receives an IP address from the switch 2 persistence table. A broadcast request is sent
out, and switch 1 responds with an unused IP address from its pool. The response can override the
assigned port.
• Switch 1 has ports FA1…FA3 in its persistence table
• Switch 2 has ports FA4, FA5, FA6, and FA8 in its persistence table
• DHCP Snooping is selected
• Reserved Only is checked
A new device that is connected to switch 1 FA1 receives an IP address from the Switch 1 persistence table.
A broadcast request is not sent across the network, so Switch 2 does not respond. If a device is connected
to FA7 of Switch 1, it does not receive an IP address from the switch pool because it is not defined in the
persistence table. Also, unused addresses in the pool are blocked.
• Switch 1 has ports FA1…FA3 in its persistence table
• Switch 2 has ports FA4, FA5, FA6, and FA8 in its persistence table
• DHCP Snooping is selected
• Reserved Only is not checked
A new device that is connected to switch 1 FA1 receives an IP address from the Switch 1 persistence table.
A broadcast request is not sent across the network, therefore Switch 2 does not respond. If a device is
connected to FA7 (not defined in the DHCP persistence table) of Switch 1, it receives an unassigned IP
address from the switch 1 pool.