User manual

80 Publication 1783-UM003D-EN-E - December 2009
Chapter 3
Reserve IP Addresses through DHCP Persistence
You can reserve and preassign an IP address from the IP address pool to a
specific switch port, so that a device connected to that switch port always
receives the same IP address regardless of its MAC address.
DHCP persistence is useful in networks that are set up 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 connected to
other switches (including other Stratix 8000 switches). If the switch receives a
DHCP request it will respond with any unassigned IP addresses in its pool. To
prevent this from happening, check the Reserve Only box on the DHCP
dialog box. This ensures that the switch will not respond when a request is
received.
When the DHCP persistence feature is enabled on a port and a DHCP request
is made from a connected device on that port, the switch assigns the IP
address for that port in the DHCP dialog box. It will also broadcast 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 may try and
respond. This may override the initial IP address assigned by the switch
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. Doing this blocks the broadcast of this
DHCP request so that no other server (including another Stratix 8000 or
Stratix 8300 switch with DHCP persistence enabled) will respond.
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 will function properly with this IP address, but you should
reassign a static IP address to the replaced devices.