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Difficulty: MediumDHCP Architecture and Relay Services

A client host on VLAN 10 (10.10.10.0/2410.10.10.0/24) boots up and requires an IPv4 address lease from a centralized DHCP server located at 192.168.100.50192.168.100.50. A Layer 3 switch serves as the DHCP relay agent with an IP helper address configured on SVI 10 (10.10.10.110.10.10.1). What is the correct sequence of network operations during the initial DHCP discover and offer phase?

  1. 1The client host broadcasts a DHCPDISCOVER packet on UDP port 67 within VLAN 10.
  2. 2The Layer 3 switch populates the `giaddr` field with 10.10.10.110.10.10.1 and sends a unicast DHCPDISCOVER packet to 192.168.100.50192.168.100.50 on UDP port 67.
  3. 3The DHCP server matches `giaddr` to the 10.10.10.0/2410.10.10.0/24 address pool, selects an available IP, and sends a unicast DHCPOFFER message back to 10.10.10.110.10.10.1.
  4. 4The Layer 3 switch receives the unicast DHCPOFFER and forwards it onto VLAN 10 to the requesting client on UDP port 68.

Answer

The correct order of network operations is: 1) Client host broadcasts a DHCPDISCOVER packet on UDP port 67 within VLAN 10; 2) Layer 3 switch populates the `giaddr` field with 10.10.10.110.10.10.1 and sends a unicast DHCPDISCOVER packet to 192.168.100.50192.168.100.50 on UDP port 67; 3) DHCP server matches `giaddr` to the 10.10.10.0/2410.10.10.0/24 address pool, selects an available IP, and sends a unicast DHCPOFFER message back to 10.10.10.110.10.10.1; 4) Layer 3 switch receives the unicast DHCPOFFER and forwards it onto VLAN 10 to the requesting client on UDP port 68.
The sequence begins when the host client transmits a broadcast DHCPDISCOVER frame on UDP port 67 inside VLAN 10. The Layer 3 switch (relay agent) intercepts this broadcast on SVI 10, sets the `giaddr` field to its local gateway IP (10.10.10.110.10.10.1), and unicasts the packet to the remote DHCP server at 192.168.100.50192.168.100.50 on UDP port 67. Upon receiving the unicast packet, the DHCP server evaluates `giaddr` to pick an IP address from the matching 10.10.10.0/2410.10.10.0/24 scope and sends a unicast DHCPOFFER reply back to the relay agent (10.10.10.110.10.10.1). Finally, the relay agent forwards the DHCPOFFER onto VLAN 10 where the client receives it on UDP port 68.

Step-by-Step Solution

1
Client broadcast initialization
DHCPDISCOVER broadcast generated on local VLAN 10 on UDP port 67.
Unconfigured hosts cannot unicast without an assigned IP address.
2
Relay agent interception and `giaddr` insertion
Layer 3 switch intercepts broadcast, inserts its SVI IP (10.10.10.110.10.10.1) into the `giaddr` header field, and converts the packet into a unicast payload directed to 192.168.100.50192.168.100.50 on UDP port 67.
Broadcast packets do not cross router boundaries; unicast conversion and `giaddr` tagging allow cross-subnet DHCP routing.
3
DHCP server pool matching and reply generation
DHCP server receives unicast request, examines `giaddr` (10.10.10.110.10.10.1), queries the corresponding 10.10.10.0/2410.10.10.0/24 scope, reserves an available IPv4 address, and sends a unicast DHCPOFFER back to 10.10.10.110.10.10.1.
The `giaddr` field tells the central server exactly which IP range to allocate from.
4
Relay agent local delivery
Layer 3 switch receives the unicast DHCPOFFER and transmits it onto VLAN 10 addressed to the client on UDP port 68.
The client software listens on UDP port 68 to receive incoming DHCP offer and acknowledgment responses.

Key Concept

DHCP Relay Agent Operational Packet Flow and `giaddr` Header Processing
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Topics

DHCP Architecture and Relay Services

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