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Catalyst 3750-X and 3560-X Switch Software Configuration Guide
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Chapter 24 Configuring DHCP Features and IP Source Guard
Understanding DHCP Server Port-Based Address Allocation
Understanding 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.
Configuring DHCP Server Port-Based Address Allocation
• Default Port-Based Address Allocation Configuration, page 24-26
• Port-Based Address Allocation Configuration Guidelines, page 24-26
• Enabling DHCP Server Port-Based Address Allocation, page 24-27

Default Port-Based Address Allocation Configuration

By default, DHCP server port-based address allocation is disabled.

Port-Based Address Allocation Configuration Guidelines

• Only one IP address can be assigned per port.
• Reserved addresses (preassigned) cannot be cleared by using the clear ip dhcp binding global
configuration command.
• Preassigned addresses are automatically excluded from normal dynamic IP address assignment.
Preassigned addresses cannot be used in host pools, but there can be multiple preassigned addresses
per DHCP address pool.
• To restrict assignments from the DHCP pool to preconfigured reservations (unreserved addresses are
not offered to the client and other clients are not served by the pool), you can enter the reserved-only
DHCP pool configuration command.