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Difficulty: Very hardDHCP Architecture and Relay Services

An enterprise network engineer is deploying redundant centralized DHCP servers (192.168.100.10192.168.100.10 and 192.168.100.11192.168.100.11) located in a dedicated management subnet. Workstations are partitioned across three client subnets (VLAN 20: 10.20.0.0/2410.20.0.0/24, VLAN 30: 10.30.0.0/2410.30.0.0/24, and VLAN 40: 10.40.0.0/2410.40.0.0/24) terminated on a Layer 3 core switch serving as their default gateway. Which TWO of the following statements correctly describe the technical mechanics and configuration requirements for the Layer 3 switch when relaying DHCPDISCOVER broadcast requests to the server pool?

  1. The Layer 3 switch inserts the IP address of the ingress Switch Virtual Interface (SVI) into the Gateway IP Address (giaddrgiaddr) field of the DHCP message header before forwarding.Answer
  2. The Layer 3 switch converts incoming client UDP broadcast packets received on destination port 67 into unicast packets directed to both configured DHCP server IP addresses on destination port 67.Answer
  3. C
    The Layer 3 switch rewrites the destination port of the forwarded unicast packet from UDP port 67 to UDP port 68 so that the central DHCP servers receive the request on the client listening port.
  4. D
    The IP helper commands must be applied within global configuration mode so that the relay functionality automatically intercepts broadcasts across all system VLANs simultaneously.

Answer

The correct statements are that the Layer 3 switch inserts the ingress SVI IP address into the Gateway IP Address (giaddrgiaddr) header field, and that it converts incoming client UDP broadcasts on port 67 into unicast UDP packets sent to both DHCP servers on destination port 67.
The correct statements accurately reflect standard DHCP relay behavior per RFC 2131: the relay agent fills the giaddrgiaddr field with the receiving interface's IP address to inform the central server of the client's subnet, and it translates client broadcast traffic into targeted unicast UDP packets sent to port 67 on all configured server endpoints.

Step-by-Step Solution

1
Analyze client broadcast reception at Layer 3 interface boundary.
Client broadcast DHCPDISCOVER packets sent to destination 255.255.255.255255.255.255.255 on UDP port 67 arrive at the SVI for VLAN 20 (10.20.0.110.20.0.1).
Layer 3 switches block Layer 2/3 broadcast frames by default unless configured with a relay agent service.
2
Evaluate header modification mechanics performed by the DHCP relay agent.
The relay agent populates giaddr=10.20.0.1giaddr = 10.20.0.1 using its receiving interface IP address.
The centralized DHCP server uses the giaddrgiaddr value to determine which pool scope (10.20.0.0/2410.20.0.0/24) to select for address allocation.
3
Evaluate transport and network layer packet forwarding to destination servers.
The Layer 3 switch sends unicast UDP packets targeting 192.168.100.10192.168.100.10 and 192.168.100.11192.168.100.11 using destination UDP port 67.
DHCP servers listen exclusively on UDP port 67; altering the destination port to 68 or failing to unicast to both configured helper targets breaks multi-server deployment.

Key Concept

DHCP Relay Agent Operational Mechanics and Header Modifications
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Topics

DHCP Architecture and Relay Services

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