Rockwell Automation 1783-BMxxx Stratix 5700 Ethernet Managed Switches User Manual - Manual (Page 140)

140 Rockwell Automation Publication 1783-UM004E-EN-P - June 2014
Chapter 4 Manage the Switch via the Device Manager Web Interface
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 13 - 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 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. This can override the assignment 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 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. Note that 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, and 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 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.This 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 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. Note that if a device
is connected to FA7 (not defined in the DHCP persistence table) of Switch 1, it does not receive an IP address
from the switch pool because it is not defined in the persistence table, and 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 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. Note that 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.