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After the DHCP is configured on the Publisher, the IP Phones do not register to the Subscriber Server when the Publisher Server is down in the Cisco CallManager 4.x cluster


Core Issue

There is no specification if it is recommended not to use Cisco CallManager for Dynamic Host Configuration Protocol (DHCP) services. There are some things that can be considered before you configure a Cisco CallManager Server in the cluster as the DHCP Server:

  • Cisco IP Phones are DHCP-enabled by default. If you do not use DHCP, you need to disable DHCP on the phone and manually assign it to an IP address.

  • DHCP is always enabled for Cisco Analog Access and Cisco Digital Access gateways.

  • For Cisco Catalyst 6000 8 Port Voice T1/E1 and Services Modules, the Network Management Processor (NMP) on the Catalyst 6000 can or  cannot possibly have DHCP enabled. If DHCP is not enabled, you need to configure the IP address through the Cisco IOS  command-line interface on the Cisco Catalyst 6000.

  • Users with many phones use the publisher as the TFTP and/or DHCP server, because there is not the desire for the publisher to be in charge of call processing. The subscribers are used to register the IP phones and for call processing.

  • If the DHCP server fails, you have these problems because the IP phones do not obtain an IP address. You have to be sure that your DHCP server is up all the time.

Also, if the Cisco CallManager is configured as a DHCP server for a large number of IP phones, the server becomes overloaded. All the IP phones that try to get the IP on the Cisco CallManager server together can cause some server performance and network performance issues.


As a good practice, it is better to configure the standalone DHCP Servers in the network and dedicate one server for the data and another for the voice VLAN.

The configuration of the Windows 2000 DHCP Server for Cisco CallManager has two major tasks.

  1. Create a new scope in the DHCP server.

  2.   Add a TFTP option to the scope. 

Refer to the Add the TFTP Option to the Scope section of Configuring Windows 2000 DHCP Server for Cisco CallManager for more information on how to configure the DHCP server.

Problem Type

Call control software  (CallManager, CallManager Express, ICS7750, SRST, SS7 call agents)

IP phones and other clients  (Client registration, faulty IP phone, etc.)

Call Control


CallManager Features

Bad operational behavior, status,information, performance, delay or other CCM failure

IP Phones and Clients

Registration problems

Failure Type

Cannot connect to server, disconnect, register, synchronize, communicate, etc.

End Where Problem is Heard or Device Type Involved

IP phone

IP Address Resolution


CallManager Versions

CallManager 4.x

CallManager Distributed Call Processing

CallManager server

CallManager cluster



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