Information Technology Reference
In-Depth Information
Using a router as a DHCP server is a somewhat common practice in smaller networks.
Once you move into larger organizations, DHCP services are typically centralized onto
server platforms. Either DHCP option is capable of sending TFTP server information to
the IP phones.
Example 3-3 shows the syntax used to configure a WAN branch router as a DHCP server.
Example 3-3
Configuring Router-Based DHCP Services
WAN_RTR#configure terminal
WAN_RTR(config)#ip dhcp excluded-address 172.16.1.1 172.16.1.9
WAN_RTR(config)#ip dhcp excluded-address 172.16.2.1 172.16.2.9
WAN_RTR(config)#ip dhcp pool DATA_SCOPE
WAN_RTR(dhcp-config)#network 172.16.2.0 255.255.255.0
WAN_RTR(dhcp-config)#default-router 172.16.2.1
WAN_RTR(dhcp-config)#dns-server 4.2.2.2
WAN_RTR(dhcp-config)#exit
WAN_RTR(config)#ip dhcp pool VOICE_SCOPE
WAN_RTR(dhcp-config)#network 172.16.1.0 255.255.255.0
WAN_RTR(dhcp-config)#default-router 172.16.1.1
WAN_RTR(dhcp-config)#option 150 ip 172.16.1.1
WAN_RTR(dhcp-config)#dns-server 4.2.2.2
Note:
This example uses a Cisco router as a DHCP server. I (Jeremy) took this approach
because using a router as a DHCP server is simple and stable. That being said, most people
use a Windows server or some other centralized device for DHCP services. Even Cisco
Unified Communications Manager includes DHCP server capabilities. In these cases, you
typically need to configure an
ip helper-address
<central DHCP server IP address>
to
forward DHCP requests to the central DHCP server for the voice VLAN devices.
The way in which Cisco routers approach DHCP configurations is slightly different from
how many other DHCP servers do so. Most DHCP servers allow you to specify a range of
IP addresses that you would like to hand out to clients. Cisco routers take the opposite ap-
proach: you first specify a range of addresses that you do not want to hand out to clients
(using the
ip dhcp excluded-address
syntax from global configuration mode). Configur-
ing the excluded addresses before you configure the DHCP pools ensures that the Cisco
router does not accidentally hand out IP addresses before you have a chance to exclude
them from the range. The DHCP service on the router will begin handing out IP addresses
from the first nonexcluded IP address in the network range. In Example 3-3, this is
172.16.1.10 for the voice scope and 172.16.2.10 for the data scope.
Tip:
Notice a DNS server of 4.2.2.2 is assigned to both the data and voice devices. This is
a well-known, open DNS server on the Internet. This IP address works fantastically to test
connectivity and DNS services in new network deployments because it is such a simple IP
address to remember.
