Information Technology Reference
In-Depth Information
TFTP is crucial to the correct operation of a CUCM network, so it is recommended that DHCP option 150 be used because it allows
TFTP server redundancy to be configured. Cisco also recommends that option 150 be used to specify differently ordered lists of TFTP
server addresses to hosts on different subnets to provide load balancing.
To better understand the importance of TFTP in a CUCM network and the significance of DHCP option 150, it is a good idea to
examine Cisco IP phone initialization and registration.
Cisco IP phone initialization and registration involves the following:
1.
Assuming that the phone is not using its own power supply, the phone powers itself using inline power. If the Cisco IP phone
is connected to a switch that supports power over Ethernet (PoE), the switch detects the phone and applies power to the
corresponding Ethernet port.
2.
Next, the Cisco IP phone loads a locally stored software image using a bootstrap loader.
3.
The Cisco IP phone now obtains VLAN information from the switch to which it is connected using CDPv2 packets.
4.
Assuming that the information is not statically configured, the phone attempts to obtain an IP address and other information
including TFTP server address by broadcasting a DHCP Discover message.
The DHCP server responds by sending information including an IP address, mask, TFTP server address, and default gateway
address.
The TFTP server address can be specified by the DHCP server using one of two options:
Option 66:
Specifies a TFTP server in the form of a hostname. However, it is possible to use an IP address with
option 66, and a Cisco IP phone will correctly interpret the address.
■
Option 150:
Specifies two TFTP server IP addresses.
■
5.
After the phone obtains a TFTP server address, it retrieves its configuration from the TFTP server.
The phone requests a specific configuration file corresponding to its name (SEP
mac_address
.cnf.xml). If the phone is
manually registered in CUCM, this file exists and the request will succeed. However, the request will fail if the phone is not
manually registered.
