If you want to configure a DHCP reservation on a switch or router, gather the MAC address of the device. If you are unsure of the MAC address of the device, you can find the current IP address and issue the following commands to get the MAC address and clear the DHCP binding before you create the pool.
show ip dhcp binding | include 10.11.12.4
clear ip dhcp binding 10.11.12.4
Next, run the following commands to setup the reservation. Obviously change the pool name and the IP addresses referenced to match your environment. You may notice that the client-identifier below is 14 digits rather than 12 like a standard MAC address. You have to append 01 in front of the MAC address.
ip dhcp pool My_Reservation
host 10.11.12.4 255.255.254.0