Cisco Systems 3560X Manual De Usuario

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.

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.