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Difficulty: MediumDHCP Architecture and Relay Services

A network engineer configures an IP helper address on a Layer 3 switch's VLAN interface (192.168.10.1/24192.168.10.1/24) to relay client requests to a centralized DHCP server at 10.100.1.5010.100.1.50. When a workstation on VLAN 10 broadcasts a DHCPDISCOVER message, which set of actions does the DHCP relay agent perform on the packet before sending it to the server?

  1. It populates the giaddr field with 192.168.10.1192.168.10.1, sets the source IP to its outgoing interface address, and unicasts the packet to 10.100.1.5010.100.1.50 via UDP port 67.Answer
  2. B
    It encapsulates the DHCP broadcast inside a TCP port 67 session established with 10.100.1.5010.100.1.50 to ensure reliable delivery across the routed network.
  3. C
    It changes the destination UDP port from 67 to 68 so that the central DHCP server receives the request on its listening service port.
  4. D
    It floods the unmodified Layer 2 broadcast frame across all active Layer 3 routed ports until it reaches the VLAN where the DHCP server resides.

Answer

The DHCP relay agent populates the giaddr (gateway IP address) field with the local interface address (192.168.10.1192.168.10.1), converts the client's Layer 2 broadcast into a Layer 3 unicast packet, and forwards it directly to the DHCP server (10.100.1.5010.100.1.50) using UDP destination port 67.
When a Layer 3 device functions as a DHCP relay agent (via `ip helper-address`), it intercepts client DHCPDISCOVER broadcast packets on the receiving interface. It modifies the packet by inserting its own local interface IP address into the `giaddr` (Gateway IP Address) header field and replaces the broadcast destination address with the specific unicast IP address of the DHCP server. It then forwards the packet across the routed infrastructure using standard UDP port 67.

Step-by-Step Solution

1
Analyze incoming broadcast request
The client sends a local broadcast DHCPDISCOVER frame to 255.255.255.255 on UDP port 67.
Clients lacking an IP address must broadcast locally to find a DHCP server.
2
Process and modify packet header at the relay agent
The Layer 3 switch relay agent intercepts the broadcast, inserts its own interface IP (192.168.10.1192.168.10.1) into the `giaddr` field, sets its egress IP as the source IP, and sets the destination IP to 10.100.1.5010.100.1.50.
The `giaddr` field informs the remote DHCP server which subnet scope must be used to select an available IP address.
3
Unicast packet transport across routed network
The packet is transmitted via standard Layer 3 routing to destination IP 10.100.1.5010.100.1.50 on UDP port 67.
Unicast IP packets can be forwarded across routers, whereas Layer 2 broadcasts are blocked at subnet boundaries.

Key Concept

DHCP Relay Agent and GIADDR Operation
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Topics

DHCP Architecture and Relay Services

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