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Difficulty: Very hardDHCP Server, Client, and Relay Agent Operation and Configuration

In a multi-vlan corporate topology, an end host attached to subinterface GigabitEthernet0/0.30 broadcasts a DHCP DISCOVER frame. The gateway router (R1) has subinterface GigabitEthernet0/0.30 configured with IPv4 address 192.168.30.1/24 and the ip helper-address 10.5.5.50 command pointing to a central DHCP server. When R1 processes and forwards this DHCP request to the server across the routed core, which field in the DHCP message header does R1 modify, and what specific value is inserted into that field?

  1. The Gateway IP Address (giaddr) field, populated with 192.168.30.1Answer
  2. B
    The Gateway IP Address (giaddr) field, populated with 10.5.5.50
  3. C
    The Server Host Name (sname) field, populated with 192.168.30.1
  4. D
    The Client IP Address (ciaddr) field, populated with 192.168.30.255

Answer

The correct answer specifies that the Gateway IP Address (giaddr) field is populated with 192.168.30.1.
When a router receives a broadcast DHCP message on an interface configured with `ip helper-address`, it acts as a DHCP Relay Agent. The router converts the broadcast into a unicast message sent to the IP specified in the `ip helper-address` command. Crucially, the router sets the `giaddr` (Gateway IP Address) field inside the DHCP header to the primary IPv4 address configured on the interface that received the client's broadcast (192.168.30.1). This informs the central DHCP server which IP pool/subnet to select an address from and provides the return path for the unicast DHCP OFFER.

Step-by-Step Solution

1
Analyze client broadcast reception
The router receives the layer 2/3 broadcast (255.255.255.255 / UDP 67) from the client on subinterface GigabitEthernet0/0.30.
The client initial DHCP DISCOVER message is unnumbered and must be broadcast within its local broadcast domain (VLAN 30).
2
Examine relay agent processing
Because helper address 10.5.5.50 is configured on GigabitEthernet0/0.30, the router encapsulates the packet as a unicast IPv4 packet directed to 10.5.5.50.
Routers drop broadcast packets by default; the relay agent converts the broadcast to a unicast frame to traverse routed network hops.
3
Determine GIADDR field insertion
The router writes its receiving interface IP address (192.168.30.1) into the DHCP header giaddr (Gateway IP Address) field.
The DHCP server requires the giaddr field to match the incoming client gateway network address in order to select the corresponding scope (192.168.30.0/24) and route the unicast offer back to the relay agent.

Key Concept

DHCP Relay Agent GIADDR Field Operation
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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