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Difficulty: HardIP Addressing Services

A network engineer is integrating a new remote branch office subnet (172.16.40.0/24172.16.40.0/24) connected to corporate headquarters through a Layer 3 site-to-site VPN. Client workstations at the branch fail to obtain dynamic IP configurations from the central enterprise DHCP server (10.10.10.510.10.10.5). Packet captures confirm that client DHCPDISCOVER broadcast frames arrive at the branch router's inbound local interface, but no DHCPOFFER packets are returned. Which configuration change on the branch router directly resolves this connectivity failure?

  1. Configure an IP helper address pointing to 10.10.10.510.10.10.5 on the branch router's LAN interface facing the client subnet.Answer
  2. B
    Configure an IP helper address pointing to 10.10.10.510.10.10.5 on the branch router's WAN interface facing the VPN tunnel.
  3. C
    Add a CNAME record on the headquarters DNS server mapping the branch default gateway IP to the central DHCP server hostname.
  4. D
    Enable UDP port 69 relay forwarding on the branch router's LAN interface to pass client discovery packets across the gateway.

Answer

Configure an IP helper address pointing to 10.10.10.510.10.10.5 on the branch router's LAN interface facing the client subnet.
Configuring an IP helper address on the client-facing LAN interface allows the branch router to intercept client broadcast DHCPDISCOVER messages (UDP port 67). The router converts these broadcasts into unicast IP packets targeted directly at the remote central DHCP server (10.10.10.510.10.10.5) across the routed VPN, populating the gateway IP field (GIADDR) so the server can assign an address from the 172.16.40.0/24172.16.40.0/24 scope.

Step-by-Step Solution

1
Analyze the traffic type and network boundary condition.
Client workstations issue broadcast DHCPDISCOVER requests (255.255.255.255255.255.255.255) on the local LAN. Routers drop Layer 3 broadcast traffic by default.
Broadcast packets cannot naturally traverse the Layer 3 site-to-site VPN to reach the central server at 10.10.10.510.10.10.5.
2
Determine the correct service mechanism and placement.
A DHCP Relay Agent (IP Helper) must be deployed on the router interface receiving the client broadcasts (the local LAN interface).
The relay agent intercept broadcasts on the ingress LAN interface, encapsulates them into unicast packets directed to 10.10.10.510.10.10.5, and inserts gateway IP address (GIADDR) information so the server knows which scope to allocate from.
3
Evaluate distractor configurations against technical requirements.
Placing the helper on the WAN interface, creating CNAME records, or forwarding UDP port 69 (TFTP) fails to address the root cause of unforwarded DHCP UDP 67/68 broadcasts.
Only configuring the IP helper address on the LAN interface correctly converts local incoming broadcasts to unicast packets sent to the central server.

Key Concept

DHCP Relay Agent and IP Helper Placement
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Topics

IP Addressing Services

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