Cisco IE 4000, Software Configuration Manual

Page: 400 / 1066

396
Configuring DHCP
Information About Configuring DHCP
Note: Do not enable DHCP snooping on RSPAN VLANs. If DHCP snooping is enabled on RSPAN VLANs, DHCP
packets might not reach the RSPAN destination port.
DHCP Snooping Binding Database Guidelines

Because both NVRAM and the flash memory have limited storage capacity, we recommend that you store the binding
file on a TFTP server.

For network-based URLs (such as TFTP and FTP), you must create an empty file at the configured URL before the
switch can write bindings to the binding file at that URL. See the documentation for your TFTP server to determine
whether you must first create an empty file on the server; some TFTP servers cannot be configured this way.

To ensure that the lease time in the database is accurate, we recommend that you enable and configure NTP. For
more information, see
Configuring Time and Date Manually, page 111 .

If NTP is configured, the switch writes binding changes to the binding file only when the switch system clock is
synchronized with NTP.
Packet Forwarding Address
If the DHCP server and the DHCP clients are on different networks or subnets, you must configure the switch with the ip
helper-address address interface configuration command. The general rule is to configure the command on the Layer
3 interface closest to the client. The address used in the ip helper-address command can be a specific DHCP server IP
address, or it can be the network address if other DHCP servers are on the destination network segment. Using the
network address enables any DHCP server to respond to requests.
DHCP Server Port-Based Address Allocation
DHCP server port-based address allocation is a feature that enables DHCP to maintain the same IP address on an
Ethernet switch port regardless of the attached device client identifier or client hardware address.
When Ethernet switches are deployed in the network, they offer connectivity to the directly connected devices. In some
environments, such as on a factory floor, if a device fails, the replacement device must be working immediately in the
existing network. With the current DHCP implementation, there is no guarantee that DHCP would offer the same IP
address to the replacement device. Control, monitoring, and other software expect a stable IP address associated with
each device. If a device is replaced, the address assignment should remain stable even though the DHCP client has
changed.
When configured, the DHCP server port-based address allocation feature ensures that the same IP address is always
offered to the same connected port even as the client identifier or client hardware address changes in the DHCP
messages received on that port. The DHCP protocol recognizes DHCP clients by the client identifier option in the DHCP
packet. Clients that do not include the client identifier option are identified by the client hardware address. When you
configure this feature, the port name of the interface overrides the client identifier or hardware address and the actual
point of connection, the switch port, becomes the client identifier.
In all cases, by connecting the Ethernet cable to the same port, the same IP address is allocated through DHCP to the
attached device.
The DHCP server port-based address allocation feature is only supported on a Cisco IOS DHCP server and not a
third-party server.
By default, DHCP server port-based address allocation is disabled.
Port-Based Address Allocation Configuration Guidelines
These are the configuration guidelines for DHCP port-based address allocation: