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Difficulty: HardDHCP Architecture and Relay Services

A systems engineer is configuring a Layer 3 switch to relay DHCP requests from client devices on `Vlan 40` (10.40.0.0/2410.40.0.0/24) to a centralized DHCP server located on `Vlan 100` (172.16.100.10172.16.100.10). Workstations on `Vlan 40` fail to obtain IP leases and default to APIPA addresses (169.254.x.x169.254.x.x). Packet captures confirm that client DHCPDISCOVER broadcast packets reach interface `Vlan 40`, but the switch never forwards them to 172.16.100.10172.16.100.10. Reviewing the switch configuration reveals that `ip helper-address 172.16.100.10` was applied to interface `Vlan 100` instead of interface `Vlan 40`. Which of the following statements correctly explains why client leasing is failing and identifies the necessary correction?

  1. The relay agent must be placed on the client-facing gateway interface (`Vlan 40`) so it can intercept local layer 2 broadcasts, insert its interface IP into the gateway IP address (`giaddr`) field, and forward unicast packets to the DHCP server.Answer
  2. B
    The relay agent placement on `Vlan 100` is correct, but cross-VLAN relaying fails because the relay command must be updated to convert broadcast UDP traffic into connection-oriented TCP port 67 traffic.
  3. C
    The `ip helper-address` directive is required on the server-facing interface (`Vlan 100`) to permit returning unicast DHCPOFFER packets to enter the Layer 3 switch backplane.
  4. D
    The helper command location is immaterial, but client requests fail because relay agents transmit unicast requests targeting destination UDP port 68 on the remote DHCP server instead of port 67.

Answer

The relay agent must be configured on the client-facing gateway interface (`Vlan 40`) to intercept local broadcasts, set the gateway IP address (`giaddr`) field to the switch's interface address, and unicast the DHCP request to the server.
DHCPDISCOVER messages are sent as local subnets broadcasts. A Layer 3 interface acting as the default gateway for clients must have `ip helper-address` enabled to listen for incoming UDP port 67 broadcasts on that specific interface. Once captured, the relay agent injects the incoming interface IP into the packet's `giaddr` field and forwards it via unicast routing to the designated DHCP server IP.

Step-by-Step Solution

1
Analyze client broadcast domain dynamics.
Clients on `Vlan 40` issue Layer 2 broadcast DHCPDISCOVER packets that cannot cross Layer 3 boundaries without an active relay agent on their default gateway interface.
Routers and Layer 3 switches drop Layer 2/3 broadcast frames by default.
2
Evaluate the function of the Gateway IP Address (`giaddr`) field.
When `ip helper-address` is active on `Vlan 40`, the Layer 3 device receives the client broadcast, populates the `giaddr` field with its `Vlan 40` IP address (10.40.0.110.40.0.1), and unicasts the request to 172.16.100.10172.16.100.10.
The centralized DHCP server reads the `giaddr` field to determine which address pool/scope to draw the IP lease from.
3
Identify misconfiguration and remediation.
Configuring `ip helper-address` on `Vlan 100` fails to capture `Vlan 40` broadcasts. Moving the configuration to interface `Vlan 40` restores relay functionality.
The IP Helper feature only intercepts broadcasts arriving on the specific interface where the command is bound.

Key Concept

DHCP Relay Agent Interface Placement and `giaddr` Insertion
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Topics

DHCP Architecture and Relay Services

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