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The CUCM DHCP server is designed to serve IP phones in small deployments (maximum
of 1000 devices). It provides a subset of the functionality that was provided by the Windows
2000 Server in CUCM versions earlier than CallManager 5.0.
Only one DHCP server can be configured per CUCM cluster; no backup configuration is
possible. The CUCM DHCP server can be configured with multiple subnets. DHCP relay
must be enabled in remote subnets to allow the DHCP broadcast request packets to be
forwarded to the DHCP server. Routers drop all broadcast packets by default, but the
packets can be configured with DHCP relay by using the ip helper-address command.
To configure DHCP support on CUCM, follow these steps:
Step 1 Activate the DHCP Monitor Service.
Step 2 Add and configure the DHCP server.
Step 3 Configure the DHCP subnets.
Navigate to the Cisco Unified Serviceability web pages. From the Tools menu, choose
Service Activation. Activate the DHCP Monitor Service by clicking the DHCP Monitor
Service check box and then clicking the Save icon. Figure 5-3 is a screen capture of the
DHCP Monitor Service activation.
The DHCP server then needs to be configured. Configure the DHCP server from the CUCM
Administration page. Navigate to System > DHCP > DHCP Server. The Find and List
DHCP Servers page will display. Click the Add New button. Choose the CUCM server that
will be acting as the DHCP server from the Host Server drop-down menu. Configure the
Primary TFTP Server IP Address field and the Secondary TFTP Server IP Address field. It
is advisable to have two CUCM servers running the TFTP service for fault-tolerance
purposes. Figure 5-4 shows the DHCP server configuration page options.
NOTE Due to the CPU load impact, CUCM DHCP server must not be used in deployments
larger than 1000 registered devices. The CPU load of the server can be monitored
using the Real-Time Monitoring Tool (RTMT). If high CPU load is experienced, the
DHCP service should be provided by other devices (DHCP server, switch, or router).
RTMT is covered in Cisco Unified Communications IP Telephony, Part 2.
CUCM Initial Configuration 93
Figure 5-3 Service Activation: DHCP Monitor Service
Figure 5-4 DHCP Server Configuration
94 Chapter 5: Initial Configuration Settings
The DHCP subnet(s) needs to be configured from the CUCM Administration page.
Navigate to System > DHCP > DHCP Subnet. The Find and List DHCP Subnets page will
display. Click the Add New button. Choose the DHCP server from the DHCP Server dropdown
menu. This is a required field. All fields in the configuration pages that have an
asterisk (*) to the upper right of the configuration option are required fields. Specify the
subnet IP address, IP address range, primary router IP address, subnet mask, and TFTP
servers. Figure 5-5 displays the DHCP subnet configuration page options.
Figure 5-5 DHCP Subnet Configuration
In CUCM releases earlier than 5.0, DHCP services could be provided by the Windowsbased
operating system of CUCM. If the Windows DHCP server was used and CUCM is
upgraded to a CUCM release running the Linux operating system, all DHCP configuration
is lost. The Data Migration Assistant (DMA) does not migrate Windows DHCP configuration.
DHCP can configured on CUCM servers beginning with Release 5.0.
DNS
CUCM can use either IP addresses or names to refer to other IP devices in application
settings. When names are used, they need to be resolved to IP addresses by DNS.
CUCM Initial Configuration 95
Both methods have some advantages:
■ Using IP addresses: The system does not depend on a DNS server. This prevents loss
of service when the DNS server cannot be reached. Clients, using DNS, query the
server with a name lookup request and receive a reply in which the DNS server resolves
the hostname record of the query to an IP address. Eliminating the requirement of a DNS
server reduces the danger of DNS configuration errors, DNS outages, and the latency
issues involved in sending a query and receiving a response using the DNS model.
Troubleshooting is simplified when using IP addresses rather than DNS names because
there is no need to perform name resolution.
■ Using DNS: Management is simplified because logical names are easier to remember
than IP addresses. If IP addresses change, there is no need to modify any of the application
settings that rely on the existing IP address. When DNS is used, the applications point
to a DNS name that does not change; the underlying IP address might change at any
time with no consequence to the IP addresses that rely on the server. CUCM server
addressing is sent to Cisco IP Phones in the CUCM group in the phone’s XML
configuration file. The addressing sent down to the IP phone can be based on IP
addresses or names.
Because of the additional point of failure introduced using DNS, the Cisco best
practices recommendation is to not use DNS with CUCM.
Table 5-2 summarizes the advantages and disadvantage of using DNS with CUCM.
Before the IP phone can communicate with CUCM, it has to resolve the name of the server.
Signaling messages are then exchanged between the Cisco IP Phone and CUCM, as
illustrated in Figure 5-6.
Table 5-2 IP Addressing and DNS Comparison
IP Addressing Advantages DNS Advantages
Does not require a DNS server Simplifies management because of the use of
names rather than numbers
Prevents the IP telephony network from failing if
the IP phones lose connection to the DNS server
Enables easier IP address changes because of
name-based IP paths
Decreases the amount of time required when a
device attempts to contact the Unified CM server
Allows server to IP phone NAT
Simplifies troubleshooting
96 Chapter 5: Initial Configuration Settings
Figure 5-6 Call Flow with DNS
When DNS naming is not used in the CUCM cluster, there is no need to resolve the name
of the CUCM to an IP address. The signaling between the IP phone and CUCM can be set
up faster, and calls can be processed even if the DNS service is not available. CUCM will
have higher availability and faster response times by removing any DNS reliance. Call flow
without the use of DNS is illustrated in Figure 5-7.
Figure 5-7 Call Flow Without DNS
IP Phone A IP Phone B
2) Signaling Protocol
3) RTP Media Path
2) Signaling Protocol
1) DNS Query/Response 1) DNS Query/Response
DNS Server
IP Phone A IP Phone B
1) Signaling Protocol
2) RTP Media Path
1) Signaling Protocol
Network and Feature Services 97
To change the system to operate without a DNS server, follow these steps:
Step 1 In CUCM Administration, go to System > Server.
Step 2 Click the Find button and select the first (next) available server from the
list of CUCM servers.
Step 3 Change the server name to the IP address of the server and save the
changes, as shown in Figure 5-8.