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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 25 Configuring DHCP
Information About Configuring DHCP
• You can display DHCP snooping statistics by entering the show ip dhcp snooping statistics user
EXEC command, and you can clear the snooping statist ics counters by entering the clear ip dhcp
snooping statistics privileged EXEC command.
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 the “Configuring Time and Date Manually” section on
page 7-9.
• 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 networ k 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 conne cted
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 o r client hardware
address changes in the DHCP messages rece ived on that port. The DHCP protocol recognizes DH CP
clients by the client identifier option in the DHCP packet. Clients that do not include the client identifier