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Difficulty: HardDHCP Architecture and Relay Services

A workstation on a remote branch subnet initializes network communications and requires an IP address assignment from a centralized DHCP server located across a routed WAN link. Place the chronological steps of the DHCP Relay forwarding process in the correct sequence from initial client packet creation to the reception of the initial offer.

  1. 1The client workstation originates a broadcast DHCPDISCOVER packet on the local segment using source UDP port 6868 and destination UDP port 6767.
  2. 2The local Layer 3 relay agent intercepts the broadcast frame, populates the Gateway IP Address (giaddrgiaddr) field with its own incoming interface IP, and unicasts the payload to the DHCP server using destination UDP port 6767.
  3. 3The centralized DHCP server uses the incoming giaddrgiaddr value to select the corresponding IP pool scope and returns a unicast DHCPOFFER packet to the relay agent's interface on UDP port 6767.
  4. 4The relay agent receives the unicast response from the server, strips the relay encapsulation, and transmits the DHCPOFFER packet to the workstation on the local segment targeting UDP port 6868.

Answer

The correct sequence starts with the workstation broadcasting a DHCPDISCOVER request locally on UDP port 67. Next, the Layer 3 relay agent inserts its interface IP into the giaddr field and unicasts the request to the central DHCP server. The DHCP server processes the giaddr field to match the scope and responds with a unicast DHCPOFFER to the relay agent. Finally, the relay agent forwards the DHCPOFFER down to the workstation segment on UDP port 68.
The DHCP sequence across subnets requires specific steps to bridge broadcast domains. First, the unconfigured client broadcasts a DHCPDISCOVER message using destination UDP port 6767. Second, the local relay agent captures the broadcast, inserts its local interface IP into the gateway IP address (giaddrgiaddr) header field, and unicasts the message across the routed network to the DHCP server on UDP port 6767. Third, the server uses the giaddrgiaddr value to select the proper subnet pool and unicasts a DHCPOFFER back to the relay agent interface. Fourth, the relay agent delivers the offer back to the client host on UDP port 6868.

Step-by-Step Solution

1
Identify the initial client request behavior.
The client broadcasts a DHCPDISCOVER packet locally using source UDP port 6868 and destination UDP port 6767.
Unconfigured hosts must broadcast locally to locate network configuration services.
2
Determine the action of the DHCP relay agent (IP Helper).
The relay agent captures the broadcast, appends its local interface address into the Gateway IP Address (giaddrgiaddr) field, and forwards the packet as a unicast transmission to the central DHCP server on UDP port 6767.
Routers drop local broadcasts by default; relaying requires converting the broadcast to a routable unicast packet and tagging the originating subnet.
3
Trace the DHCP server processing and reply.
The server matches the giaddrgiaddr to the appropriate scope, generates a DHCPOFFER, and unicasts it back to the relay agent interface on UDP port 6767.
The server relies on giaddrgiaddr rather than the IP header source address to determine which pool of addresses to draw from.
4
Trace the relay agent's final delivery to the client.
The relay agent forwards the DHCPOFFER packet onto the client's local subnet to UDP port 6868.
The client listens for DHCP server replies on UDP port 6868 to parse lease parameters.

Key Concept

DHCP Relay Agent Unicast Conversion and giaddr Field Processing
Estimated Time:2m 0s
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Topics

DHCP Architecture and Relay Services

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