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Difficulty: MediumDHCP Architecture and Relay Services

A workstation on a remote branch subnet requests an IPv4 address from a centralized DHCP server located across a routed WAN link via a local Layer 3 DHCP relay agent. Arrange the steps of the initial DHCP discovery and offer phase in the correct chronological order from start to finish.

  1. 1The workstation broadcasts a DHCPDISCOVER message on UDP port 67 within its local broadcast domain.
  2. 2The relay agent intercepts the broadcast, populates the gateway IP address (giaddr) field with its ingress interface IP, and unicasts the packet to the central DHCP server on UDP port 67.
  3. 3The central DHCP server selects an IP pool matching the giaddr subnet and unicasts a DHCPOFFER packet back to the relay agent on UDP port 68.
  4. 4The relay agent receives the unicast offer and forwards it to the client's local broadcast domain on UDP port 68.
  5. 5The workstation receives the offer and broadcasts a DHCPREQUEST message to accept the offered configuration parameters.

Answer

The correct sequence is: 1) Workstation broadcasts DHCPDISCOVER locally, 2) Relay agent populates giaddr and unicasts DHCPDISCOVER to the server, 3) DHCP server selects scope via giaddr and unicasts DHCPOFFER to relay agent, 4) Relay agent forwards DHCPOFFER to the local subnet, 5) Workstation broadcasts DHCPREQUEST.
The standard DHCP relay packet flow begins when an unconfigured host broadcasts a DHCPDISCOVER packet locally on UDP port 67. The local router acting as a relay agent receives this broadcast, stamps its own interface address into the Gateway IP Address (giaddr) field, and unicasts the packet to the centralized DHCP server on UDP port 67. Upon receiving the packet, the DHCP server inspects giaddr to identify the appropriate IP pool, picks an available IP address, and unicasts a DHCPOFFER back to the relay agent on UDP port 68. The relay agent then forwards the DHCPOFFER onto the local subnet to the host on UDP port 68. Finally, the host processes the offer and broadcasts a DHCPREQUEST to formally select the offered address.

Step-by-Step Solution

1
Identify the initial client transmission
Workstation broadcasts a DHCPDISCOVER frame locally on UDP port 67.
Clients lacking an IP address must broadcast locally to find a server or relay agent.
2
Trace the relay agent behavior upon packet ingress
Relay agent modifies packet with giaddr field and unicasts it to the server.
Layer 3 boundaries block broadcast frames; the relay converts broadcast to unicast so routing works.
3
Determine central server processing and egress response
Server uses giaddr to select the correct IP scope and unicasts DHCPOFFER to the relay agent.
The server must know which subnet the client belongs to, which is indicated by the giaddr field.
4
Trace relay agent forwarding back to client
Relay agent receives DHCPOFFER unicast and forwards it onto the client's local subnet.
The relay agent bridges communication between the remote server and local host.
5
Identify client response to complete the offer acceptance phase
Workstation broadcasts a DHCPREQUEST accepting the IP lease.
In the standard DORA sequence, DISCOVER is followed by OFFER, then REQUEST.

Key Concept

DHCP Relay Agent Operational Packet Flow (DORA across L3 boundaries)
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Topics

DHCP Architecture and Relay Services

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