Specifications

Setting Up the Network
How to Set Up the Network
36
Cisco IOS Survivable Remote Site Telephony Version 3.4 System Administrator Guide
Configuring DHCP for Cisco SRST Phones
To perform this task, you must have your network configured with DHCP. For further details about
DHCP configuration, see the Cisco IOS DHCP Server document and refer to your Cisco CallManager
documentation.
When a Cisco IP phone is connected to the Cisco SRST system, it automatically queries for a DHCP
server. The DHCP server responds by assigning an IP address to the Cisco IP phone and providing the
IP address of the TFTP server through DHCP option 150. Then the phone registers with the
Cisco CallManager system server and attempts to get configuration and phone firmware files from the
Cisco CallManager TFTP server address provided by the DHCP server.
When setting up your network, configure your DHCP server local to your site. You may use your SRST
router to provide DHCP service (recommended). If your DHCP server is across the WAN and there is
an extended WAN outage, the DHCP lease times on your Cisco IP phones may expire. This may cause
your phones to lose their IP addresses, resulting in a loss of service. Rebooting your phones when there
is no DHCP server available after the DHCP lease has expired will not reactivate the phones, because
they will be unable to obtain an IP address or other configuration information. Having your DHCP server
local to your remote site ensures that the phones can continue to renew their IP address leases in the
event of an extended WAN failure.
Choose one of the following tasks to set up DHCP service for your IP phones:
• Defining a Single DHCP IP Address Pool, page 36—Use this method if the Cisco SRST router is a
DHCP server and if you can use a single shared address pool for all your DHCP clients.
• Defining a Separate DHCP IP Address Pool for Each Cisco IP Phone, page 37—Use this method if
the Cisco SRST router is a DHCP server and you need separate pools for non-IP-phone DHCP
clients.
• Defining the DHCP Relay Server, page 38—Use this method if the Cisco SRST router is not a
DHCP server and you want to relay DHCP requests from IP phones to a DHCP server on a different
router.
Defining a Single DHCP IP Address Pool
This task creates a large shared pool of IP addresses in which all DHCP clients receive the same
information, including the option 150 TFTP server IP address. The benefit of selecting this method is
that you set up only one DHCP pool. However, defining a single DHCP IP address pool can be a problem
if some (non-IP phone) clients need to use a different TFTP server address.
SUMMARY STEPS
1. ip dhcp pool pool-name
2. network ip-address [mask | prefix-length]
3. option 150 ip ip-address
4. default-router ip-address
5. exit