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Difficulty: Very hardIP Addressing Services

An enterprise network engineer is deploying Voice over IP (VoIP) telephony across multiple remote subnets. Devices on VLAN 10 receive IP addresses and function normally. However, newly deployed IP phones residing on VLAN 30 successfully acquire IPv4 addresses, default gateway settings, and DNS server addresses from the central DHCP server, but remain stuck in a boot loop reporting 'Server Unreachable' when attempting to download their configuration files. Analysis confirms that IP routing between VLAN 30 and the TFTP/provisioning server is fully functional. Which of the following DHCP service configurations on the central server is missing or misconfigured for the VLAN 30 scope?

  1. The custom vendor-specific DHCP option (Option 66 or Option 150) specifying the IP address or hostname of the provisioning server is missing from the DHCP scope for VLAN 30.Answer
  2. B
    The DHCP relay agent statement must be configured on the central DHCP server's network interface to forward client TFTP request packets to VLAN 30.
  3. C
    The primary forward lookup zone on the DNS server lacks an AAAA record mapping the provisioning server's hostname to its IPv4 address.
  4. D
    The DHCP relay agent port mapping must be converted from UDP port 67 to TCP port 68 to permit binary payload transfers during phone initialization.

Answer

The custom vendor-specific DHCP option (Option 66 or Option 150) specifying the TFTP/provisioning server IP address or hostname is missing from the VLAN 30 DHCP scope.
When VoIP endpoints boot up, basic IP address allocation is insufficient for full operation. IP phones rely on specific DHCP scope options, such as Option 66 (TFTP Server Name) or Option 150 (Cisco proprietary option for TFTP server IP addresses), to locate the server containing their operational firmware and configuration files. Because the phones acquired basic IP details but fail specifically at file download, the missing component is the DHCP option pointing to the provisioning server.

Step-by-Step Solution

1
Analyze the symptoms described in the scenario.
IP phones receive standard IP parameters (IP address, gateway, DNS) via DHCP and IP connectivity to the TFTP server exists, but phones cannot locate the configuration server.
Standard DHCP parameters (Options 3 and 6) do not automatically provide vendor-specific provisioning server locations to VoIP devices.
2
Identify the required IP addressing service configuration for automated VoIP provisioning.
DHCP Option 66 (TFTP Server Name) or Option 150 (TFTP Server IP Address) must be defined within the specific DHCP scope.
IP phones require these options to discover where to download firmware and initial configuration files upon completing the basic DHCP lease process.

Key Concept

DHCP Options and Vendor-Specific Boot Configuration
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Topics

IP Addressing Services

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