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Difficulty: Very hardIP Addressing Services

A network administrator is troubleshooting an issue where new IP phones on VLAN 50 (10.50.0.0/2410.50.0.0/24) successfully obtain IPv4 leased addresses and default gateway settings from a centralized DHCP server located on VLAN 10 (10.10.0.10010.10.0.100), but repeatedly fail to locate their TFTP provisioning server (10.10.0.20010.10.0.200) and fail to sync time with the corporate NTP server (10.10.0.1010.10.0.10). The layer 3 switch interface for VLAN 50 is configured with `ip helper-address 10.10.0.100`. Packet captures on the switch show DHCP requests are forwarded, but TFTP and NTP requests sent via broadcast during initial boot are dropped. Which configuration modification on the layer 3 switch will directly resolve the provisioning and time synchronization failure for the endpoints on VLAN 50?

  1. Configure additional UDP forward port commands (`ip forward-protocol udp 69` and `ip forward-protocol udp 123`) or configure DHCP scope Options 66 and 42 on the central DHCP server.Answer
  2. B
    Add a second `ip helper-address 10.10.0.200` entry under the VLAN 50 interface to relay DHCPDISCOVER broadcasts to the TFTP server host.
  3. C
    Create a canonical name (CNAME) record on the internal DNS server mapping the default hostname `tftp.local` to the IPv6 address of the provisioning server.
  4. D
    Reassign the centralized DHCP server service port from UDP port 67 to TCP port 68 to bypass layer 3 switch broadcast filtering policies.

Answer

Configure additional UDP forward port commands or configure DHCP scope Options 66 and 42 on the central DHCP server.
Centralized network services across routed subnets require remote hosts to receive unicast destination information or require layer 3 devices to relay specific broadcast protocols. Configuring DHCP scope Option 66 (TFTP server) and Option 42 (NTP server) delivers unicast IPv4 addresses directly to endpoints during initial DHCP address leasing. Alternatively, ensuring `ip forward-protocol udp` rules allow UDP broadcast translation for ports 69 and 123 allows legacy broadcast discovery to reach centralized servers.

Step-by-Step Solution

1
Analyze client boot sequence requirements
IP phones require IP address configuration (via DHCP), TFTP server address for boot file download (Option 66), and NTP server address for time synchronization (Option 42).
Endpoints on remote subnets cannot discover centralized services via local layer 2 broadcasts across routed boundaries.
2
Evaluate relay behavior of `ip helper-address`
The `ip helper-address` command relays broadcast UDP traffic destined for specific ports to targeted unicast destinations.
If clients broadcast to discover TFTP (port 69) or NTP (port 123) without receiving DHCP scope options, UDP broadcast forwarding must be explicitly permitted or DHCP scope options must supply unicast addresses.
3
Select the correct infrastructure configuration solution
Providing DHCP Scope Options 66 and 42 ensures clients receive unicast target addresses directly, while adjusting UDP forward rules permits necessary relay traffic.
This directly satisfies the requirement for endpoints on VLAN 50 to locate and communicate with services on VLAN 10.

Key Concept

DHCP Relay (IP Helper) Operations and Service Option Configurations
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Topics

IP Addressing Services

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