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Difficulty: HardDHCP Architecture and Relay Services

A client workstation on VLAN 20 initiates a connection and requests an IP address assignment from a centralized DHCP server residing on VLAN 100 via a Layer 3 switch interface configured with DHCP relay. Place the following stages of the relayed DHCPDISCOVER and DHCPOFFER packet lifecycle in the correct chronological sequence from first to last.

  1. 1The host workstation broadcasts a DHCPDISCOVER packet on UDP port 67 within local VLAN 20.
  2. 2The Layer 3 switch interface receives the broadcast packet and populates the giaddrgiaddr (Gateway IP Address) field with its own VLAN 20 SVI address.
  3. 3The Layer 3 switch encapsulates the request and transmits a unicast DHCPDISCOVER packet across VLAN 100 to the central DHCP server's IP address.
  4. 4The DHCP server selects the VLAN 20 address scope matching giaddrgiaddr and transmits a unicast DHCPOFFER packet back to the relay agent's interface IP.
  5. 5The Layer 3 switch relay agent forwards the DHCPOFFER packet onto VLAN 20 to be received by the requesting workstation.

Answer

The correct chronological sequence is: (1) Host broadcasts DHCPDISCOVER locally on VLAN 20, (2) Relay agent populates the giaddr field with the VLAN 20 SVI address, (3) Relay agent forwards unicast DHCPDISCOVER to the server on VLAN 100, (4) Server selects scope based on giaddr and sends unicast DHCPOFFER to the relay agent, (5) Relay agent delivers DHCPOFFER back to the host on VLAN 20.
The correct sequence reflects standard DHCP relay operations across Layer 3 boundaries. The process begins when the client generates a broadcast DHCPDISCOVER packet on its local segment. The Layer 3 relay interface receives the broadcast, tags its own interface IP into the giaddrgiaddr payload field, and forwards the packet as a unicast transmission to the remote DHCP server. The DHCP server uses the giaddrgiaddr to map the request to the correct scope, then unicasts a DHCPOFFER back to the relay interface IP. Finally, the relay interface delivers the DHCPOFFER to the local client on VLAN 20.

Step-by-Step Solution

1
Identify client-side initiation
The host generates a local broadcast DHCPDISCOVER packet on UDP port 67 because it lacks an IP address.
Broadcast frames are contained within VLAN 20 and cannot traverse Layer 3 boundaries without a relay agent.
2
Relay agent interception and giaddr insertion
The SVI on VLAN 20 intercepts the packet and inserts its SVI IP into the giaddrgiaddr field.
The giaddrgiaddr field informs the remote server which logical subnet requested the lease.
3
Unicast forwarding to server
The relay agent routes a unicast packet to the designated DHCP server IP across the routing core.
Converts non-routable broadcast traffic to routable IP unicast traffic.
4
Server scope evaluation and unicast response
DHCP server matches giaddrgiaddr to the VLAN 20 scope and sends a unicast DHCPOFFER back to the relay agent address.
The server uses the giaddrgiaddr as the destination IP address for the DHCPOFFER reply.
5
Client delivery
The relay agent receives the DHCPOFFER and relays it onto the client's access segment.
Completes the initial discovery phase of the DORA process across subnets.

Key Concept

DHCP Relay Agent Architecture & GIADDR Mechanics
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Topics

DHCP Architecture and Relay Services

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