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Question

Difficulty: MediumDHCP Architecture and Relay Services

A network engineer is configuring a centralized DHCP server (10.100.1.5010.100.1.50) to service client devices across multiple remote subnets. A workstation located on the Sales subnet (192.168.15.0/24192.168.15.0/24) broadcasts a DHCPDISCOVER message, which is intercepted by the local default gateway router. How does the central DHCP server determine which specific IP address scope to allocate the lease from?

  1. The server evaluates the Gateway IP Address (giaddr) field in the DHCP header, which was populated by the relay agent with its local interface IP address.Answer
  2. B
    The server inspects the source MAC address field in the Ethernet header to map the client to its physical switchport VLAN broadcast domain.
  3. C
    The server receives the original broadcast frame encapsulated in TCP port 67 and extracts the client's requested IP address option.
  4. D
    The server matches the destination IP address of the incoming unicast packet, which is set to the client's default gateway IP address.

Answer

The server evaluates the Gateway IP Address (giaddr) field in the DHCP header, which was populated by the relay agent with its local incoming interface IP address.
When a Layer 3 device functions as a DHCP relay agent (e.g., using `ip helper-address`), it receives the client's local broadcast DHCPDISCOVER packet and rewrites it into a unicast packet destined for the central DHCP server. Crucially, the relay agent fills the Gateway IP Address (`giaddr`) field in the DHCP header with the IP address of the local interface on which the broadcast was received. Upon receiving the relayed packet, the central DHCP server inspects the `giaddr` value to select an IP address from the pool corresponding to that specific subnet.

Step-by-Step Solution

1
Analyze the client DHCPDISCOVER broadcast packet.
The client issues a broadcast packet with source IP 0.0.0.00.0.0.0 and destination IP 255.255.255.255255.255.255.255 on UDP port 67.
Clients requesting an initial IP address lease do not yet possess an IP address and broadcast to find available DHCP servers.
2
Examine the DHCP relay agent processing on the Layer 3 interface.
The router acting as a DHCP relay agent intercepts the broadcast, inserts its own interface IP address (192.168.15.1192.168.15.1) into the giaddr field, and changes the packet to a unicast packet targeting the central server (10.100.1.5010.100.1.50).
Centralized DHCP servers reside outside the local broadcast domain and require the giaddr field to identify the client's subnet.
3
Evaluate how the DHCP server matches the incoming request to a pool.
The DHCP server reads 192.168.15.1192.168.15.1 from the giaddr field and selects an available lease from the matching 192.168.15.0/24192.168.15.0/24 scope.
The giaddr field explicitly informs the server which subnet pool must be used to generate the offer.

Key Concept

DHCP Relay Gateway IP Address (giaddr) Field Scope Selection
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Topics

DHCP Architecture and Relay Services

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