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Difficulty: MediumDHCP Architecture and Relay Services

A client host connected to VLAN 20 requires an IPv4 address lease from a centralized DHCP server located on VLAN 10. The router acting as the default gateway for VLAN 20 is configured with a DHCP relay agent. What is the correct chronological sequence of network events during the initial discovery and offer process?

  1. 1The client host broadcasts a DHCPDISCOVER packet on VLAN 20 using UDP source port 68 and destination port 67.
  2. 2The router's SVI on VLAN 20 intercepts the local broadcast DHCPDISCOVER packet.
  3. 3The relay agent inserts its own VLAN 20 IP address into the giaddr field and unicasts the packet to the DHCP server on VLAN 10.
  4. 4The DHCP server evaluates the giaddr field, selects an available address from the VLAN 20 pool, and unicasts a DHCPOFFER back to the relay agent.
  5. 5The relay agent receives the unicast DHCPOFFER and forwards it onto VLAN 20 to reach the requesting client host.

Answer

The process begins when the client broadcasts a DHCPDISCOVER packet on VLAN 20 (item 1). The Layer 3 router interface on VLAN 20 intercepts the broadcast (item 2). Next, the relay agent populates the giaddr field with its own interface address and forwards the request as a unicast packet to the DHCP server on VLAN 10 (item 3). The DHCP server reads the giaddr, allocates an IP from the VLAN 20 pool, and unicasts a DHCPOFFER back to the relay agent (item 4). Finally, the relay agent transmits the DHCPOFFER onto VLAN 20 for the client host (item 5).
The correct order follows the standard DHCP relay packet flow across Layer 3 boundaries. The process starts with a local client broadcast on VLAN 20, followed by interception by the router interface relay agent. The relay agent sets its local IP in the giaddr field and unicasts the packet to the DHCP server on VLAN 10. The server uses the giaddr to select the VLAN 20 scope and unicasts a DHCPOFFER back to the relay agent, which delivers it onto VLAN 20 for the client.

Step-by-Step Solution

1
Identify the initial client-generated broadcast packet.
The client emits a broadcast DHCPDISCOVER on UDP port 67 within VLAN 20.
Unconfigured hosts must broadcast locally to find available DHCP servers or relay agents.
2
Identify Layer 3 gateway interception.
The router interface (acting as relay agent) traps the broadcast frame on VLAN 20.
Routers drop broadcast packets by default unless a DHCP relay feature (such as ip helper-address) is configured.
3
Determine relay packet modification and unicast forwarding.
The router sets the giaddr header field to its VLAN 20 IP address and unicasts the packet across routed links to the server IP on VLAN 10.
The giaddr field informs the remote server which IP subnet scope to assign from.
4
Determine server processing and response generation.
The DHCP server selects a VLAN 20 lease scope based on giaddr and unicasts a DHCPOFFER back to the relay agent's giaddr address.
The server responds via unicast to the relay agent gateway.
5
Complete relay forwarding to the requesting client.
The relay agent receives the server's unicast DHCPOFFER and relays it onto VLAN 20 to the client host.
The client receives the offered IP configurations on its local Layer 2 network segment.

Key Concept

DHCP Relay Agent Operational Sequence and giaddr Field Insertion
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Topics

DHCP Architecture and Relay Services

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