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Difficulty: Very hardDHCP Architecture and Relay Services

A network engineer is configuring a newly provisioned Layer 3 switch interface to relay DHCP traffic from a client subnet (10.100.20.0/2410.100.20.0/24) to a centralized DHCP server located at 10.200.10.510.200.10.5. Although the command `ip helper-address 10.200.10.5` has been applied to the client-facing VLAN interface, workstations on the subnet fail to obtain IP addresses. Packet captures on the centralized DHCP server indicate that relayed unicast packets arrive, but the server discards them because the Gateway IP Address (giaddrgiaddr) field in the DHCP header is set to 0.0.0.00.0.0.0. Which of the following configuration issues is the primary cause of this behavior?

  1. The client-facing VLAN interface on the Layer 3 switch lacks an assigned IPv4 address, preventing the relay agent from identifying the client subnet scope.Answer
  2. B
    The IP helper address was configured on the egress interface connecting toward the centralized DHCP server rather than the client gateway interface.
  3. C
    The switch modified the destination UDP port from port 67 to port 68 when converting the broadcast frame into a unicast packet.
  4. D
    The Layer 3 switch failed to flood the broadcast frame across all ports member to the management VLAN prior to encapsulation.

Answer

The client-facing VLAN interface on the Layer 3 switch lacks an assigned IPv4 address, preventing the relay agent from identifying the client subnet scope.
DHCP Relay agents require a valid Layer 3 IPv4 address on the client-facing interface where the broadcast is received. This interface IP address is written into the giaddrgiaddr field of the DHCP header before forwarding the packet via unicast to the DHCP server. If the interface lacks an IP address, the relay agent sets giaddrgiaddr to 0.0.0.00.0.0.0, causing the DHCP server to discard the request because it cannot determine which address pool/scope to allocate from.

Step-by-Step Solution

1
Analyze the role of the Gateway IP Address (giaddrgiaddr) field in DHCP relay operation.
The giaddrgiaddr field tells the DHCP server which subnet/pool the client belongs to when the request originates from a different broadcast domain.
Without a valid giaddrgiaddr, the server cannot perform scope matching for cross-subnet clients.
2
Determine how the relay agent populates the giaddrgiaddr field.
The relay agent inserts the IP address of its own interface receiving the client's original broadcast message.
This IP address represents the default gateway of the client subnet.
3
Evaluate the symptom (giaddr=0.0.0.0giaddr = 0.0.0.0).
If the receiving interface does not have an IP address configured, the relay agent cannot insert a valid gateway address into the header.
An unconfigured or unassigned interface IP address leaves giaddrgiaddr initialized as 0.0.0.00.0.0.0, causing the server to reject scope matching.

Key Concept

DHCP Relay Agent Subnet Selection via GIADDR Field
Estimated Time:2m 0s
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Topics

DHCP Architecture and Relay Services

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