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Difficulty: MediumDHCP Architecture and Relay Services

A network engineer is implementing DHCP Relay Agent Information Option (Option 82) on Layer 2 access switches within an enterprise building. When a client sends a broadcast DHCPDISCOVERDHCPDISCOVER message, the relay agent appends sub-options to the request before forwarding it to a centralized DHCP server across a routed core. Which sub-option field specifically carries identifying information about the physical switch port and VLAN from which the client request originated?

  1. Circuit ID (Sub-option 1)Answer
  2. B
    Gateway IP Address (giaddr)
  3. C
    Remote ID (Sub-option 2)
  4. D
    Client Hardware Address (chaddr)

Answer

Circuit ID (Sub-option 1) is added by the DHCP relay agent to specify the client's physical port and VLAN information.
DHCP Option 82 allows a relay agent to attach location-specific metadata to client DHCP broadcasts. Sub-option 1 (Circuit ID) encodes the specific physical interface, module, and VLAN tag where the client broadcast was received.

Step-by-Step Solution

1
Analyze the role of DHCP Option 82 (Relay Agent Information Option)
Option 82 allows relay agents to insert network topology details into client DHCP requests before forwarding them to the server.
This information enables the DHCP server to enforce security policies and assign IP addresses or parameters based on physical location.
2
Differentiate between Option 82 standard sub-options
Sub-option 1 (Circuit ID) identifies the incoming port/VLAN circuit, whereas Sub-option 2 (Remote ID) identifies the relay agent switch/router identity.
Circuit ID contains specific access port details (e.g., interface index, VLAN tag) necessary to identify where the host connects.

Key Concept

DHCP Option 82 and Relay Agent Information Sub-options
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Topics

DHCP Architecture and Relay Services

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