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Difficulty: MediumDHCP Architecture and Relay Services

A network administrator is deploying a DHCP relay service on a Layer 3 boundary router to service workstations located on VLAN 40 (172.28.40.0/24172.28.40.0/24). The centralized corporate DHCP server is located on a distant management network at 10.50.1.10010.50.1.100. Workstations on VLAN 40 are currently getting APIPA addresses (169.254.x.x169.254.x.x) because broadcast DHCPDISCOVER messages are stopped at the router. Which TWO of the following requirements or configuration steps are necessary on the router to allow clients on VLAN 40 to successfully obtain leases from the central DHCP server? (Select TWO.)

  1. Apply the IP helper address command targeting 10.50.1.10010.50.1.100 specifically on the router interface or SVI directly attached to VLAN 40.Answer
  2. Ensure the router interface facing VLAN 40 has an active IPv4 address assigned in the 172.28.40.0/24172.28.40.0/24 subnet to populate the Gateway IP Address (giaddr) field.Answer
  3. C
    Configure the helper address command on the outbound WAN interface pointing toward the central DHCP server network.
  4. D
    Configure access control lists on the router to allow incoming TCP port 67 and TCP port 68 traffic from VLAN 40 workstations.

Answer

The administrator must apply the IP helper address command targeting 10.50.1.10010.50.1.100 directly on the VLAN 40 interface/SVI, and ensure that the interface facing VLAN 40 has an active IPv4 address in the 172.28.40.0/24172.28.40.0/24 subnet to populate the giaddr header field.
To successfully relay DHCP traffic across Layer 3 boundaries, the router must intercept client broadcast traffic on the local client-facing interface (VLAN 40) where the `ip helper-address` command is applied. Additionally, the router uses the IP address configured on that client-facing interface to fill the `giaddr` (Gateway IP Address) field in the DHCP packet header. This field informs the remote DHCP server which subnet scope to select for assigning an IP address lease.

Step-by-Step Solution

1
Identify the ingress boundary interface for client DHCP broadcasts.
The router interface/SVI connected directly to VLAN 40 is identified as the ingress point for broadcast DHCPDISCOVER frames.
Routers drop Layer 2 broadcasts by default. The relay agent must listen on the incoming interface receiving the broadcasts.
2
Configure the DHCP relay statement on the client-facing interface.
Applying `ip helper-address 10.50.1.100` on the VLAN 40 interface enables the conversion of incoming broadcasts to unicast packets sent to the server.
This tells the router where to forward unicast DHCP requests across routing boundaries.
3
Verify giaddr field population requirements.
The router inserts its own IP address from the VLAN 40 interface into the `giaddr` field of the unicast DHCP packet.
The centralized DHCP server reads the `giaddr` value to determine which IP address pool (172.28.40.0/24172.28.40.0/24) to issue a lease from.

Key Concept

DHCP Relay Agent Operation and Gateway IP Address (giaddr) Insertion
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Topics

DHCP Architecture and Relay Services

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