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Difficulty: MediumDHCP Server, Client, and Relay Agent Operation and Configuration

A client host on VLAN 20 (172.16.20.0/24172.16.20.0/24) broadcasts a DHCPDISCOVER request to obtain network parameters. The default gateway router R1 forwards this request to a centralized DHCP server at 10.200.1.10010.200.1.100 via an `ip helper-address` statement configured on subinterface GigabitEthernet0/0.20. Which field inside the relayed DHCP payload does R1 populate with its own ingress subinterface IP address to inform the DHCP server which address pool to use?

  1. Gateway IP Address (giaddr)Answer
  2. B
    Client IP Address (ciaddr)
  3. C
    Your IP Address (yiaddr)
  4. D
    Client Hardware Address (chaddr)

Answer

The Relay Agent populates the Gateway IP Address (giaddr) field with its ingress interface IP address.
When a router acts as a DHCP relay agent via the `ip helper-address` command, it receives the client's broadcast DHCPDISCOVER message, inserts its own receiving interface IP address into the Gateway IP Address (giaddr) field, and forwards the packet as a unicast message to the targeted DHCP server. The DHCP server relies on the giaddr field to identify the originating subnet and allocate an IP address from the correct pool.

Step-by-Step Solution

1
Analyze the role of the DHCP relay agent when receiving a broadcast request.
The router receives a Layer 2/3 broadcast DHCPDISCOVER on subinterface GigabitEthernet0/0.20 (172.16.20.1172.16.20.1).
DHCP clients initially broadcast requests because they do not possess an IP address or know the server location.
2
Examine how the helper address processes and encapsulates the packet.
The router converts the broadcast into a unicast packet directed to 10.200.1.10010.200.1.100.
The `ip helper-address` configuration triggers the router to forward the request across Layer 3 boundaries.
3
Identify the specific field used to convey subnet origin context to the server.
The router inserts its interface address (172.16.20.1172.16.20.1) into the `giaddr` field.
The DHCP server reads `giaddr` to match and allocate an available IP address from the corresponding 172.16.20.0/24172.16.20.0/24 pool.

Key Concept

DHCP Relay Agent Operational Mechanics and giaddr Field Population
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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