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Difficulty: EasyDHCP Server, Client, and Relay Agent Operation and Configuration

Place the following operational steps in the correct chronological order to show how a Cisco router configured as a DHCP relay agent processes an incoming client discovery message.

  1. 1The host client broadcasts a DHCP DISCOVER packet on its local subnet.
  2. 2The local router interface receives the broadcast packet and evaluates the configured ip helper-address.
  3. 3The router inserts its local receiving interface IP address into the Gateway IP Address (giaddr) field.
  4. 4The router converts the packet to a unicast IP packet and forwards it to the target DHCP server address.

Answer

The correct sequence begins with the host client broadcasting a DHCP DISCOVER packet on its local subnet. Next, the local router interface receives the broadcast packet and evaluates the configured ip helper-address. The router then inserts its local receiving interface IP address into the Gateway IP Address (giaddr) field. Finally, the router converts the packet to a unicast IP packet and forwards it to the target DHCP server address.
When an unconfigured host requests an IP address, it broadcasts a DHCP DISCOVER frame. The local router interface intercepts this broadcast, modifies the DHCP header by adding its receiving interface IP to the giaddr field, and changes the destination from broadcast to unicast directed at the configured DHCP server IP.

Step-by-Step Solution

1
Identify the client-side initiation of the DHCP process.
The host client generates a broadcast DHCP DISCOVER message.
Unconfigured hosts must use broadcast addressing to discover available DHCP servers on the local segment.
2
Determine the initial reception step by the relay agent.
The router interface captures the broadcast and checks for an active ip helper-address statement.
The ip helper-address command instructs the interface to process incoming UDP port 67 broadcast traffic.
3
Identify the mandatory payload modification performed by the relay agent.
The router writes its own interface IP into the packet's giaddr field.
Without the giaddr field populated, the central server cannot determine which pool and subnet address range to offer.
4
Determine the final packet forwarding operation.
The router forwards the modified DHCP request as a unicast packet to the central DHCP server.
Broadcasts are not routed by default; relaying requires converting the frame into a unicast transmission targeting the DHCP server.

Key Concept

DHCP Relay Agent Operation and Payload Modification (giaddr)
Estimated Time:45s
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Topics

DHCP Server, Client, and Relay Agent Operation and Configuration

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