Information Technology Reference
In-Depth Information
Issue 3: DHCP Server
Receiving an IP address through DHCP goes hand-in-hand with the voice VLAN assign-
ment (Issue 2). If a Cisco IP Phone is not assigned to the correct VLAN, it may not receive
an IP address from the DHCP server, or if it does, the DHCP options for the pool may not
be correct. After you verify the voice VLAN configuration, you can use this process to
troubleshoot the DHCP process:
Check the DHCP helper-address: If you are using a centralized DHCP server,
ensure a router (or L3 switch) supporting the voice VLAN is forwarding DHCP re-
quests to a proper server. (You can find the helper-address command under the router
interface connected to the VLAN.)
Check the DHCP server: Verify that the DHCP server has an IP address pool cre-
ated for the voice VLAN devices, ensure that the pool has not run out of IP ad-
dresses, verify that DHCP option 150 (TFTP Server) is properly configured and
assigned to the pool, connect other test devices (laptop or PC) in the voice VLAN,
and ensure these devices receive IP addresses.
Check the IP phone: If you have physical access to the IP phone, navigate to the
network settings page (using the Settings button on the phone). Verify that the IP
phone received an IP address from the appropriate subnet, verify all applicable DHCP
options (subnet mask, default gateway, TFTP server) are filled in, and attempt to ping
the phone from another subnet to ensure routing works (assuming no access control
lists [ACL] block this communication).
Keep in mind that it's easy to mix up DHCP-related troubleshooting with other phone sys-
tem issues. Because a phone experiencing communication issues constantly reboots, there
are times when the phone does not have an IP address (which can send you down a wrong
track of troubleshooting). Before you pull your hair out focusing on DHCP issues, try
statically assigning an IP configuration to the phone and see if the phone registers suc-
cessfully with CME. If it does register successfully, the problem is most likely related to
DHCP or VLAN issues. If not, the problem is more likely related to routing, TFTP, or
CME issues.
Issue 4: TFTP Server
The TFTP server is a critical part of the IP phone boot process because it supplies the
phone firmware and configuration file with the base settings the phone should use for op-
eration (and the IP address of the CME server for registration). Although CME supports
using an external TFTP server to store all this data, most CME deployments simply use
the flash memory and dynamic RAM of the router to store these files. Take the following
steps to troubleshoot TFTP communication:
Check routing configuration: If the TFTP server is on a different subnet than the IP
phone, validate that data is able to route between the two subnets by placing a test
devoice (such as a laptop or PC) in the voice VLAN and testing connectivity to the
TFTP server (by transferring files via TFTP).
Check the TFTP server: Verify that the TFTP server is operational and serving
files, validate the firmware for the IP phone model in question exists on the TFTP
 
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