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Difficulty: EasyDHCP Architecture and Relay Services

A workstation on a remote VLAN requires an IP address assignment from a central DHCP server located on a different subnet across a Layer 3 router. What is the correct sequence of packet delivery steps during the discovery and offering stage of this relayed DHCP request?

  1. 1The client host broadcasts a DHCPDISCOVER packet on its local VLAN subnet.
  2. 2The router interface functioning as the DHCP relay agent receives the broadcast, inserts its gateway IP address (giaddr), and unicasts the DHCPDISCOVER packet to the central DHCP server.
  3. 3The central DHCP server selects an IP address pool matching the giaddr and replies to the relay agent with a unicast DHCPOFFER packet.
  4. 4The DHCP relay agent forwards the DHCPOFFER packet onto the local VLAN subnet for the requesting host.

Answer

The sequence begins when the client broadcasts a DHCPDISCOVER packet on its local subnet. The DHCP relay agent receives this broadcast, inserts its own interface address as the gateway IP (giaddr), and forwards the packet as a unicast message to the central DHCP server. The DHCP server matches the giaddr to the appropriate IP scope and returns a unicast DHCPOFFER packet to the relay agent. Finally, the relay agent forwards the DHCPOFFER packet to the local subnet for the client host.
In a cross-subnet DHCP environment, the host initiates the process by broadcasting a DHCPDISCOVER packet locally. The relay agent on the router interface receives this broadcast, populates the giaddr header field with its interface IP, and forwards the packet as a unicast message to the central DHCP server. The server reads the giaddr, allocates an IP from the corresponding scope, and returns a unicast DHCPOFFER packet to the relay agent. The relay agent then forwards the DHCPOFFER message onto the host's subnet.

Step-by-Step Solution

1
Identify the initial client request broadcast.
The unconfigured host initiates communication by broadcasting a DHCPDISCOVER message within its local broadcast domain.
Clients lacking IP configurations rely on local broadcast packets to reach DHCP listening services.
2
Determine how the relay agent processes the broadcast request.
The configured router interface intercepts the broadcast, inserts its interface IP into the giaddr field, and sends a unicast DHCPDISCOVER packet to the remote server IP.
Routers block standard broadcast packets by default; the relay agent encapsulation enables cross-subnet transmission.
3
Trace the server response mechanism back to the relay agent.
The DHCP server identifies the subnet pool via the giaddr header and responds with a unicast DHCPOFFER packet addressed to the relay agent.
The server cannot reply directly to the unconfigured client's unrouted IP, so it targets the reachable relay agent interface.
4
Trace the final delivery to the client.
The relay agent forwards the DHCPOFFER packet onto the client's local subnet segment.
This completes the delivery of offered lease details to the requesting workstation host.

Key Concept

DHCP Relay Agent Packet Processing and Relay Sequence
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Topics

DHCP Architecture and Relay Services

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