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Difficulty: EasyDHCP Architecture and Relay Services

When a network router configured as a DHCP relay agent receives an incoming DHCPDISCOVER broadcast message from a client host, how does it modify the packet header before forwarding it to a remote DHCP server?

  1. It sets the Gateway IP Address (GIADDR) field to the IP address of the router interface that received the broadcast.Answer
  2. B
    It alters the destination port of the packet from UDP port 67 to UDP port 68.
  3. C
    It converts the transport layer protocol from UDP to TCP to ensure connection-oriented delivery across subnets.
  4. D
    It forwards the request as a Layer 2 broadcast frame across all active interfaces on the router.

Answer

The router sets the Gateway IP Address (GIADDR) field to the IP address of the receiving interface before forwarding the packet as a unicast message to the DHCP server.
When a router acts as a DHCP relay agent, it intercepts broadcast DHCPDISCOVER messages on a local network interface and populates the Gateway IP Address (GIADDR) field with that interface's IP address. It then unicasts the packet to the remote DHCP server, allowing the server to determine which pool of addresses to draw from based on the GIADDR subnet.

Step-by-Step Solution

1
Identify the primary role of a DHCP Relay Agent during client address discovery.
The relay agent intercepts the client's Layer 2/Layer 3 broadcast (DHCPDISCOVER) on the local subnet.
Broadcast frames cannot cross Layer 3 router boundaries without explicit relay processing.
2
Examine the packet header modifications made by the relay agent.
The relay agent records the IP address of its receiving interface into the GIADDR (Gateway IP Address) field of the DHCP message header.
The remote DHCP server uses the GIADDR value to identify the originating subnet and assign an appropriate IP address from the correct scope.
3
Determine the forwarding behavior across the network.
The router converts the client broadcast into a Layer 3 unicast packet destined for the configured DHCP server IP address, keeping the destination port as UDP port 67.
Unicast forwarding allows the packet to be routed across intermediate network subnets to reach the centralized DHCP server.

Key Concept

DHCP Relay GIADDR Field Population
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Topics

DHCP Architecture and Relay Services

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