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Difficulty: MediumDHCP Architecture and Relay Services

A network engineer is configuring a centralized DHCP server located at 10.200.1.10/2410.200.1.10/24 to issue IPv4 address leases to workstations across remote subnets. The remote site utilizes a Layer 3 router as the default gateway for multiple local networks, including VLAN 15 (10.15.0.0/2410.15.0.0/24). Workstations connected to VLAN 15 are failing to receive IP addresses and are instead generating APIPA addresses. Packet captures indicate that client DHCP DISCOVER broadcast messages from VLAN 15 are not reaching the centralized DHCP server across the WAN link. Which configuration modification on the remote router will resolve this connectivity issue?

  1. Configure the helper address command with 10.200.1.1010.200.1.10 specifically on the VLAN 15 interface on the remote router.Answer
  2. B
    Configure the helper address command with 10.200.1.1010.200.1.10 on the WAN uplink interface facing the central core network.
  3. C
    Modify the DHCP relay traffic parameters on the router to forward requests using TCP port 67 instead of UDP port 67.
  4. D
    Reconfigure the router to forward incoming client DHCP DISCOVER messages to destination UDP port 68.

Answer

Configure the helper address command with 10.200.1.1010.200.1.10 specifically on the VLAN 15 interface on the remote router.
Configuring the IP helper address on the client-facing gateway interface allows the router to receive local broadcast DHCP DISCOVER messages from VLAN 15 hosts. The router converts these broadcasts into unicast UDP packets directed to the central DHCP server at 10.200.1.1010.200.1.10, adding the router's interface IP to the GIADDR field so the server allocates an IP from the correct 10.15.0.0/2410.15.0.0/24 pool.

Step-by-Step Solution

1
Analyze client traffic flow and broadcast boundary behavior.
Workstations on VLAN 15 issue Layer 2 and Layer 3 local broadcasts (UDP port 67) during the DHCP DISCOVER phase, which routers drop at the subnet boundary by default.
Centralized DHCP servers residing across Layer 3 boundaries cannot receive un-relayed broadcast traffic.
2
Determine the proper relay placement and operation.
The DHCP Relay Agent (IP helper address) must be enabled on the client-facing gateway interface (VLAN 15 SVI or subinterface).
This enables the router to listen for incoming client broadcasts on that specific segment, convert the broadcast into a unicast frame addressed to 10.200.1.1010.200.1.10, and populate the GIADDR header field with 10.15.0.110.15.0.1 so the server selects the correct scope.

Key Concept

DHCP Relay Agent and IP Helper Placement
Estimated Time:1m 30s
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Topics

DHCP Architecture and Relay Services

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