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Difficulty: HardDHCP Architecture and Relay Services

A network security administrator investigates an issue where client devices on VLAN 30 (172.16.30.0/24172.16.30.0/24) fail to obtain IP leases from a centralized DHCP server located in the management subnet (172.16.100.0/24172.16.100.0/24). Packet captures at the DHCP server reveal that incoming unicast DHCPDISCOVER packets correctly feature the Layer 3 switch interface address (172.16.30.1172.16.30.1) in the `giaddr` (Gateway IP Address) field; however, the server silently drops the requests due to a policy requiring switch port and VLAN attachment metadata for scope allocation. Which feature must be configured on the relaying Layer 3 switch to resolve this issue?

  1. Enable DHCP Option 82 (Relay Agent Information Option) insertion on the switch to attach circuit-id and remote-id metadata to client requests.Answer
  2. B
    Configure the `ip helper-address` on the destination interface connected directly to the centralized DHCP server rather than on the client VLAN 30 interface.
  3. C
    Change the destination protocol port mapping on the relay agent from UDP port 67 to TCP port 67 to support encrypted policy header transmission.
  4. D
    Remove VLAN 30 from the switch trunk ports to allow Layer 2 broadcast frames from client workstations to cross router boundaries directly.

Answer

Enable DHCP Option 82 (Relay Agent Information Option) insertion on the switch to attach circuit-id and remote-id metadata to client requests.
DHCP Option 82 (Relay Agent Information Option) allows a Layer 3 relay switch to insert specific location information (such as the incoming switch port number, VLAN ID, or module identifier) into client DHCP requests before relaying them to a centralized DHCP server. When enterprise DHCP servers enforce policies based on physical switch port locations, Option 82 must be enabled on the relay agent so the server can evaluate these sub-options and allocate the appropriate IP lease.

Step-by-Step Solution

1
Analyze the reported issue and packet capture evidence.
DHCPDISCOVER packets arrive at the DHCP server via unicast with `giaddr` set to 172.16.30.1172.16.30.1, confirming basic relay forwarding functions, but are dropped due to missing port/VLAN metadata required by server policy.
The basic `ip helper-address` populates `giaddr`, but advanced scope selection or access policies on centralized DHCP servers often demand physical client binding information.
2
Identify the protocol extension responsible for appending switch port and VLAN metadata.
DHCP Option 82 (Relay Agent Information Option) inserts sub-options such as Circuit ID (VLAN/interface) and Remote ID (MAC/switch identifier).
Option 82 allows the relay agent to inject context about where the client is physically attached into the DHCP request payload before forwarding it to the server.
3
Evaluate required configuration changes on the Layer 3 switch.
Enabling Option 82 insertion on the relay switch ensures metadata is appended without requiring changes to client-facing IP helper placement or transport protocols.
This satisfies the server policy requirement and resolves the scope allocation failure.

Key Concept

DHCP Option 82 (Relay Agent Information Option)
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Topics

DHCP Architecture and Relay Services

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