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Difficulty: MediumDHCP Architecture and Relay Services

A host workstation on VLAN 10 requires an IP address assignment from a centralized DHCP server located across a Layer 3 router boundary. Place the operational steps of the cross-subnet DHCP relay process in the correct sequential order from initial client discovery to final lease confirmation.

  1. 1The host workstation broadcasts a DHCPDISCOVER packet on its local VLAN 10 network segment.
  2. 2The Layer 3 relay agent populates the giaddr (gateway IP address) field with its local interface IP address and unicasts the DHCPDISCOVER packet to the central DHCP server.
  3. 3The central DHCP server evaluates the giaddr field, selects an available IP address from the corresponding scope, and unicasts a DHCPOFFER packet to the relay agent.
  4. 4The relay agent receives the unicast DHCPOFFER packet and delivers it onto VLAN 10 for the host workstation.
  5. 5The host workstation transmits a DHCPREQUEST packet to accept the offer, which the relay agent forwards to the central server to elicit the final DHCPACK confirmation.

Answer

The correct operational sequence begins with the host workstation broadcasting a DHCPDISCOVER packet on VLAN 10. The relay agent then populates the giaddr field with its own interface IP address and unicasts the packet to the central DHCP server. Next, the DHCP server evaluates the giaddr field, selects an IP from the matching scope, and unicasts a DHCPOFFER back to the relay agent. The relay agent forwards the DHCPOFFER onto VLAN 10 to the host workstation. Finally, the host sends a DHCPREQUEST, which the relay agent forwards to the server to receive the final DHCPACK confirmation.
The correct operational sequence follows the standard four-way DHCP process (DORA) modified by a Layer 3 relay agent: (1) The host workstation broadcasts a DHCPDISCOVER packet on VLAN 10. (2) The relay agent populates the giaddr header field with its interface IP address and unicasts the packet to the central DHCP server. (3) The DHCP server inspects the giaddr value, selects an IP from the corresponding subnet pool, and returns a unicast DHCPOFFER to the relay agent. (4) The relay agent delivers the DHCPOFFER onto VLAN 10. (5) The host transmits a DHCPREQUEST to select the offered parameters, which the relay agent forwards to elicit the final DHCPACK confirmation from the server.

Step-by-Step Solution

1
Identify client discovery initiation
The host workstation broadcasts a DHCPDISCOVER frame onto local VLAN 10.
An unconfigured client lacks an assigned IP address and must broadcast to discover available DHCP resources.
2
Trace relay agent header modification and unicast forwarding
The relay agent inserts its local ingress interface IP into the giaddr header field and unicasts the DHCPDISCOVER packet to the remote DHCP server.
Layer 3 boundaries block broadcast traffic by default; embedding the giaddr allows the central server to identify the client's subnet and route replies.
3
Determine server lease allocation and reply
The central DHCP server reads the giaddr value, selects an available address from the matching subnet pool, and unicasts a DHCPOFFER to the relay agent.
The server relies on giaddr to select the appropriate scope rather than the physical interface on which the unicast packet arrived.
4
Trace relay agent response forwarding to client
The relay agent receives the unicast DHCPOFFER and delivers it onto VLAN 10.
The relay agent translates the remote unicast server reply for delivery on the local client network segment.
5
Complete the request and acknowledgement exchange
The host client transmits a DHCPREQUEST packet, which is relayed to the server to trigger the final DHCPACK confirmation.
This completes the standard four-step DORA exchange conducted across a Layer 3 relay agent.

Key Concept

DHCP Relay Agent Packet Processing and Gateway IP Address (giaddr) Forwarding Sequence
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Topics

DHCP Architecture and Relay Services

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